- [ ] 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
```

}));
            }
        });
        configManager.addRegistries(tmpRegistries);
        
        List<ProtocolConfig> tmpProtocols = new ArrayList<>();
        Set<String> protocolIds = configManager.getProtocolIds();
        protocolIds.forEach(id -> {
            if (tmpProtocols.stream().noneMatch(prot -> prot.getId().equals(id))) {
                tmpProtocols.add(configManager.getProtocol(id).orElseGet(() -> {
                    ProtocolConfig protocolConfig = new ProtocolConfig();
                    protocolConfig.setId(id);
                    protocolConfig.refresh();
                    return protocolConfig;
                }));
            }
        });
        configManager.addProtocols(tmpProtocols);
    }
    /**
     * Initialize {@link #metadataService WritableMetadataService} from {@link WritableMetadataService}'s extension
     */
    private void initMetadataService() {
        this.metadataService = WritableMetadataService.getExtension(getMetadataType());
    }
    /**
     * Initialize {@link #metadataServiceExporters MetadataServiceExporter}
     */
    private void initMetadataServiceExports() {
        this.metadataServiceExporters = getExtensionLoader(MetadataServiceExporter.class).getSupportedExtensionInstances();