fix: handle NPE when Nacos NotifyCenter.INSTANCE is null during graceful shutdown

[Senrian]

What

When Spring context closes, NacosGracefulShutdownDelegate calls autoServiceRegistration.stop() which internally deregisters subscribers from Nacos NotifyCenter. However, if the Nacos client shutdown hooks have already destroyed NotifyCenter.INSTANCE, this results in:

java.lang.NullPointerException: Cannot read field "sharePublisher" because "com.alibaba.nacos.common.notify.NotifyCenter.INSTANCE" is null
  at com.alibaba.nacos.common.notify.NotifyCenter.deregisterSubscriber(NotifyCenter.java:242)
  at com.alibaba.nacos.client.naming.remote.gprc.NamingGrpcClientProxy.shutdown(NamingGrpcClientProxy.java:537)
  at com.alibaba.nacos.client.naming.remote.NamingClientProxyDelegate.shutdown(NamingClientProxyDelegate.java:212)
  at com.alibaba.nacos.client.naming.NacosNamingService.shutDown(NacosNamingService.java:637)
  at com.alibaba.cloud.nacos.NacosServiceManager.nacosServiceShutDown(NacosServiceManager.java:117)
  at com.alibaba.cloud.nacos.registry.NacosServiceRegistry.close(NacosServiceRegistry.java:120)

Fix

Wrap the stop() call in an inner try-catch to gracefully handle this race condition by logging a warning instead of propagating the error.

Testing

• Existing NacosGracefulShutdownDelegateTests continue to pass

Related Issues

Fixes #4247
Fixes #4081

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[uuuyuqi]

Thanks for your contribution. Please note that you need to sign the CLA first. Before we proceed with the review, I'd like to confirm if you have verified this? Does the issue still persist in the current nacos-client:3.1.1?