- [ ] 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
```

*
     * @param serviceInstance {@link ServiceInstance the service instance}
     * @since 2.7.8
     */
    private void preRegisterServiceInstance(ServiceInstance serviceInstance) {
        customizeServiceInstance(serviceInstance);
    }
    /**
     * Customize {@link ServiceInstance the service instance}
     *
     * @param serviceInstance {@link ServiceInstance the service instance}
     * @since 2.7.8
     */
    private void customizeServiceInstance(ServiceInstance serviceInstance) {
        ExtensionLoader<ServiceInstanceCustomizer> loader =
                getExtensionLoader(ServiceInstanceCustomizer.class);
        
        loader.getSupportedExtensionInstances().forEach(customizer -> {
            
            customizer.customize(serviceInstance);
        });
    }
    private URL selectMetadataServiceExportedURL() {
        URL selectedURL = null;
        SortedSet<String> urlValues = metadataService.getExportedURLs();
        for (String urlValue : urlValues) {
            URL url = URL.valueOf(urlValue);
            if (MetadataService.class.getName().equals(url.getServiceInterface())) {
                continue;
            }