- [ ] 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
```

import org.apache.dubbo.config.bootstrap.builders.ProviderBuilder;
import org.apache.dubbo.config.bootstrap.builders.ReferenceBuilder;
import org.apache.dubbo.config.bootstrap.builders.RegistryBuilder;
import org.apache.dubbo.config.bootstrap.builders.ServiceBuilder;
import org.apache.dubbo.config.context.ConfigManager;
import org.apache.dubbo.config.utils.ConfigValidationUtils;
import org.apache.dubbo.config.utils.ReferenceConfigCache;
import org.apache.dubbo.event.EventDispatcher;
import org.apache.dubbo.event.EventListener;
import org.apache.dubbo.event.GenericEventListener;
import org.apache.dubbo.metadata.MetadataService;
import org.apache.dubbo.metadata.MetadataServiceExporter;
import org.apache.dubbo.metadata.WritableMetadataService;
import org.apache.dubbo.metadata.report.MetadataReportInstance;
import org.apache.dubbo.registry.client.DefaultServiceInstance;
import org.apache.dubbo.registry.client.ServiceDiscovery;
import org.apache.dubbo.registry.client.ServiceDiscoveryRegistry;
import org.apache.dubbo.registry.client.ServiceInstance;
import org.apache.dubbo.registry.client.ServiceInstanceCustomizer;
import org.apache.dubbo.registry.support.AbstractRegistryFactory;
import org.apache.dubbo.rpc.model.ApplicationModel;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Set;
import java.util.SortedSet;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;