- [ ] 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
```

public static final String DEFAULT_REGISTRY_ID = "REGISTRY#DEFAULT";
    public static final String DEFAULT_PROTOCOL_ID = "PROTOCOL#DEFAULT";
    public static final String DEFAULT_SERVICE_ID = "SERVICE#DEFAULT";
    public static final String DEFAULT_REFERENCE_ID = "REFERENCE#DEFAULT";
    public static final String DEFAULT_PROVIDER_ID = "PROVIDER#DEFAULT";
    public static final String DEFAULT_CONSUMER_ID = "CONSUMER#DEFAULT";
    private static final String NAME = DubboBootstrap.class.getSimpleName();
    private final Logger logger = LoggerFactory.getLogger(getClass());
    private static volatile DubboBootstrap instance;
    private final AtomicBoolean awaited = new AtomicBoolean(false);
    private final Lock lock = new ReentrantLock();
    private final Condition condition = lock.newCondition();
    private final Lock destroyLock = new ReentrantLock();
    private final ExecutorService executorService = newSingleThreadExecutor();
    private final EventDispatcher eventDispatcher = EventDispatcher.getDefaultExtension();
    private final ExecutorRepository executorRepository = getExtensionLoader(ExecutorRepository.class).getDefaultExtension();
    private final ConfigManager configManager;
    private final Environment environment;