- [ ] 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: 2.7.8-SNAPSHOT
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

1. Do not start zookeeper server, which is used as a metadata center
2. start a provider

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

The app exit normally

### Actual Result

The app exit with a NPE

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

```
Exception in thread "DubboShutdownHook" java.lang.RuntimeException: java.lang.NullPointerException
	at org.apache.dubbo.common.function.ThrowableAction.execute(ThrowableAction.java:48)
	at org.apache.dubbo.common.lang.ShutdownHookCallbacks.lambda$callback$0(ShutdownHookCallbacks.java:70)
	at java.base/java.lang.Iterable.forEach(Iterable.java:75)
	at org.apache.dubbo.common.lang.ShutdownHookCallbacks.callback(ShutdownHookCallbacks.java:70)
	at org.apache.dubbo.config.DubboShutdownHook.callback(DubboShutdownHook.java:85)
	at org.apache.dubbo.config.DubboShutdownHook.run(DubboShutdownHook.java:73)
Caused by: java.lang.NullPointerException
	at org.apache.dubbo.config.bootstrap.DubboBootstrap.unexportMetadataService(DubboBootstrap.java:1009)
	at org.apache.dubbo.config.bootstrap.DubboBootstrap.destroy(DubboBootstrap.java:1188)
	at org.apache.dubbo.config.bootstrap.DubboBootstrap$1.callback(DubboBootstrap.java:198)
	at org.apache.dubbo.common.function.ThrowableAction.execute(ThrowableAction.java:46)
	... 5 more
```

}
        });
    }
    private void unreferServices() {
        if (cache == null) {
            cache = ReferenceConfigCache.getCache();
        }
        asyncReferringFutures.forEach(future -> {
            if (!future.isDone()) {
                future.cancel(true);
            }
        });
        asyncReferringFutures.clear();
        cache.destroyAll();
    }
    private void registerServiceInstance() {
        if (CollectionUtils.isEmpty(getServiceDiscoveries())) {
            return;
        }
        ApplicationConfig application = getApplication();
        String serviceName = application.getName();
        URL exportedURL = selectMetadataServiceExportedURL();
        String host = exportedURL.getHost();
        int port = exportedURL.getPort();
        ServiceInstance serviceInstance = createServiceInstance(serviceName, host, port);
        preRegisterServiceInstance(serviceInstance);
        getServiceDiscoveries().forEach(serviceDiscovery -> serviceDiscovery.register(serviceInstance));
    }
    /**
     * Pre-register {@link ServiceInstance the service instance}