启动报warn日志<error state, exporter should not be null>，项目启动正常。

- [*] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 3.0.3
* Operating System version: centos 7.5
* Java version: 1.8

### Steps to reproduce this issue

dubbo 3.0.3 启动项目时报warn，详细日志见下堆栈。

### Expected Result
启动异常

### Actual Result
启动无异常

What actually happens?

If there is an exception, please attach the exception trace:

```
error state, exporter should not be null
java.lang.IllegalStateException: error state, exporter should not be null
  at org.apache.dubbo.registry.integration.RegistryProtocol$OverrideListener.doOverrideIfNecessary(RegistryProtocol.java:731)
  at org.apache.dubbo.registry.integration.RegistryProtocol$ProviderConfigurationListener.lambda$notifyOverrides$0(RegistryProtocol.java:824)
  at java.util.concurrent.ConcurrentHashMap$ValuesView.forEach(ConcurrentHashMap.java:4707)
  at org.apache.dubbo.registry.integration.RegistryProtocol$ProviderConfigurationListener.notifyOverrides(RegistryProtocol.java:824)
  at org.apache.dubbo.registry.integration.AbstractConfiguratorListener.process(AbstractConfiguratorListener.java:100)
  at org.apache.dubbo.configcenter.support.zookeeper.CacheListener.lambda$dataChanged$1(CacheListener.java:107)
  at java.util.concurrent.CopyOnWriteArrayList.forEach(CopyOnWriteArrayList.java:891)
  at java.util.concurrent.CopyOnWriteArraySet.forEach(CopyOnWriteArraySet.java:404)
  at org.apache.dubbo.configcenter.support.zookeeper.CacheListener.dataChanged(CacheListener.java:107)
  at org.apache.dubbo.remoting.zookeeper.curator.CuratorZookeeperClient$NodeCacheListenerImpl.nodeChanged(CuratorZookeeperClient.java:345)
  at org.apache.curator.framework.recipes.cache.NodeCache$4.apply(NodeCache.java:326)
  at org.apache.curator.framework.recipes.cache.NodeCache$4.apply(NodeCache.java:320)
  at org.apache.curator.framework.listen.ListenerContainer$1.run(ListenerContainer.java:100)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  at java.lang.Thread.run(Thread.java:748)
```