- [ ] 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
```

if ("rest".equals(url.getProtocol())) { 
                selectedURL = url;
                break;
            } else {
                selectedURL = url; 
            }
        }
        if (selectedURL == null && CollectionUtils.isNotEmpty(urlValues)) {
            selectedURL = URL.valueOf(urlValues.iterator().next());
        }
        return selectedURL;
    }
    private void unregisterServiceInstance() {
        if (serviceInstance != null) {
            getServiceDiscoveries().forEach(serviceDiscovery -> {
                serviceDiscovery.unregister(serviceInstance);
            });
        }
    }
    private ServiceInstance createServiceInstance(String serviceName, String host, int port) {
        this.serviceInstance = new DefaultServiceInstance(serviceName, host, port);
        setMetadataStorageType(serviceInstance, getMetadataType());
        return this.serviceInstance;
    }
    public void destroy() {
        if (destroyLock.tryLock()) {
            try {
                DubboShutdownHook.destroyAll();
                if (started.compareAndSet(true, false)
                        && destroyed.compareAndSet(false, true)) {