- [ ] 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
```

.forEach(MetadataServiceExporter::export);
    }
    private void unexportMetadataService() {
        metadataServiceExporters
                .stream()
                .filter(this::supports)
                .forEach(MetadataServiceExporter::unexport);
    }
    private boolean supports(MetadataServiceExporter exporter) {
        return exporter.supports(getMetadataType());
    }
    private void exportServices() {
        configManager.getServices().forEach(sc -> {
            
            ServiceConfig serviceConfig = (ServiceConfig) sc;
            serviceConfig.setBootstrap(this);
            if (exportAsync) {
                ExecutorService executor = executorRepository.getServiceExporterExecutor();
                Future<?> future = executor.submit(() -> {
                    sc.export();
                    exportedServices.add(sc);
                });
                asyncExportingFutures.add(future);
            } else {
                sc.export();
                exportedServices.add(sc);
            }
        });
    }
    private void unexportServices() {