Exception caught from Dubbo Admin

```java
2022-11-25 20:16:56.951 ERROR 32001 --- [-20880-thread-3] o.a.dubbo.rpc.filter.ExceptionFilter : [DUBBO] Got unchecked and undeclared exception which called by fd00:1:5:5200:46d0:8a8f:9b6e:92bf. service: org.apache.dubbo.metadata.MetadataService, method: getSubscribedURLs, exception: java.lang.NullPointerException: null, dubbo version: 3.1.2, current host: 30.221.144.114, error code: 5-36. This may be caused by , go to https://dubbo.apache.org/faq/5/36 to find instructions.

java.lang.NullPointerException: null
at org.apache.dubbo.registry.client.metadata.MetadataServiceDelegation.getAllUnmodifiableSubscribedURLs(MetadataServiceDelegation.java:117)
at org.apache.dubbo.registry.client.metadata.MetadataServiceDelegation.getSubscribedURLs(MetadataServiceDelegation.java:88)
at org.apache.dubbo.registry.client.metadata.MetadataServiceDelegationDubboWrap1.invokeMethod(MetadataServiceDelegationDubboWrap1.java)
at org.apache.dubbo.rpc.proxy.javassist.JavassistProxyFactory$1.doInvoke(JavassistProxyFactory.java:73)
at org.apache.dubbo.rpc.proxy.AbstractProxyInvoker.invoke(AbstractProxyInvoker.java:100)
at org.apache.dubbo.config.invoker.DelegateProviderMetaDataInvoker.invoke(DelegateProviderMetaDataInvoker.java:55)
```

Start at org/apache/dubbo/registry/client/metadata/MetadataServiceDelegation.java:117 and trace the getAllUnmodifiableSubscribedURLs call from getSubscribedURLs. Determine why the NullPointerException occurs for the reported MetadataService request, then verify that the request no longer fails and that the reported stack trace is addressed.

Written by the indexing model from the issue text.