spring-cloud-gateway use sentinel client error

## Issue Description
```java
[02:09:07:58:02:261] ERROR 37 @@HttpServer@@ | [id: 0x6a08ab3a, L:/x.x.x.x:19920 - R:/x.x.x.x:29953]
reactor.core.Exceptions$BubblingException: java.lang.NullPointerException
at reactor.core.Exceptions.bubble(Exceptions.java:154) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators.onErrorDropped(Operators.java:520) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at com.alibaba.csp.sentinel.adapter.reactor.InheritableBaseSubscriber.onError(InheritableBaseSubscriber.java:179) ~[sentinel-reactor-adapter-1.6.3.jar!/:?]
at com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber.onError(SentinelReactorSubscriber.java:37) ~[sentinel-reactor-adapter-1.6.3.jar!/:?]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondError(MonoFlatMap.java:185) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapInner.onError(MonoFlatMap.java:251) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onError(FluxDoFinally.java:129) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onError(MonoFlatMap.java:165) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondError(MonoFlatMap.java:185) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapInner.onError(MonoFlatMap.java:251) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.onError(Operators.java:1556) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreInner.onError(MonoIgnoreThen.java:235) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onError(FluxDoFinally.java:129) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators.error(Operators.java:181) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:48) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDoFinally.subscribe(MonoDoFinally.java:47) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:3848) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:172) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:150) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:275) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:849) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:114) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1515) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:144) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:114) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:275) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:849) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:73) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:192) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1515) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFilterWhen$MonoFilterWhenMain.onNext(MonoFilterWhen.java:140) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2071) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFilterWhen$MonoFilterWhenMain.onSubscribe(MonoFilterWhen.java:103) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFilterWhen.subscribe(MonoFilterWhen.java:56) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeek.subscribe(MonoPeek.java:71) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:3848) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:442) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onNext(FluxConcatMap.java:244) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onNext(FluxDematerialize.java:114) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onNext(FluxDematerialize.java:42) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:243) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:201) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.request(FluxDematerialize.java:157) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:229) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDematerialize$DematerializeSubscriber.onSubscribe(FluxDematerialize.java:88) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:139) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:63) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDematerialize.subscribe(FluxDematerialize.java:39) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:54) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:121) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoNext.subscribe(MonoNext.java:40) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoMap.subscribe(MonoMap.java:55) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoSwitchIfEmpty.subscribe(MonoSwitchIfEmpty.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoMap.subscribe(MonoMap.java:55) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:3848) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:442) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:212) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:139) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:63) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:121) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoNext.subscribe(MonoNext.java:40) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoSwitchIfEmpty.subscribe(MonoSwitchIfEmpty.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDoFinallyFuseable.subscribe(MonoDoFinallyFuseable.java:48) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:150) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1515) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:247) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:329) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2071) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoZip$ZipInner.onSubscribe(MonoZip.java:318) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:3848) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoZip.subscribe(MonoZip.java:128) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at com.alibaba.csp.sentinel.adapter.reactor.MonoSentinelOperator.subscribe(MonoSentinelOperator.java:40) ~[sentinel-reactor-adapter-1.6.3.jar!/:?]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeekFuseable.subscribe(MonoPeekFuseable.java:74) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.Mono.subscribe(Mono.java:3848) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:172) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeekFuseable.subscribe(MonoPeekFuseable.java:70) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61) ~[reactor-core-3.2.10.RELEASE.jar!/:3.2.10.RELEASE]
at reactor.netty.http.server.HttpServerHandle.onStateChange(HttpServerHandle.java:64) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at reactor.netty.tcp.TcpServerBind$ChildObserver.onStateChange(TcpServerBind.java:226) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at reactor.netty.http.server.HttpServerOperations.onInboundNext(HttpServerOperations.java:442) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:91) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at reactor.netty.http.server.HttpTrafficHandler.channelRead(HttpTrafficHandler.java:161) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:93) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at reactor.netty.http.server.AccessLogHandler.channelRead(AccessLogHandler.java:51) [reactor-netty-0.8.9.RELEASE.jar!/:0.8.9.RELEASE]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:438) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:323) [netty-codec-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:297) [netty-codec-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930) [netty-transport-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:796) [netty-transport-native-epoll-4.1.36.Final-linux-x86_64.jar!/:4.1.36.Final]
at io.netty.channel.epoll.AbstractEpollChannel$AbstractEpollUnsafe$1.run(AbstractEpollChannel.java:385) [netty-transport-native-epoll-4.1.36.Final-linux-x86_64.jar!/:4.1.36.Final]
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163) [netty-common-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:405) [netty-common-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:338) [netty-transport-native-epoll-4.1.36.Final-linux-x86_64.jar!/:4.1.36.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:906) [netty-common-4.1.36.Final.jar!/:4.1.36.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-common-4.1.36.Final.jar!/:4.1.36.Final]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_222]
Caused by: java.lang.NullPointerException
```

### Tell us your environment
com.alibaba.cloud:spring-cloud-starter-alibaba-sentinel:2.1.0.RELEASE

Start by reproducing the Spring Cloud Gateway failure represented by the stack trace, focusing on com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber and InheritableBaseSubscriber. Determine the cause of the NullPointerException and add a regression test demonstrating the corrected behavior; done means the reported request no longer produces this error.

Written by the indexing model from the issue text.