- [ ] 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
```

private ReferenceConfigCache cache;
    private volatile boolean exportAsync;
    private volatile boolean referAsync;
    private AtomicBoolean initialized = new AtomicBoolean(false);
    private AtomicBoolean started = new AtomicBoolean(false);
    private AtomicBoolean ready = new AtomicBoolean(true);
    private AtomicBoolean destroyed = new AtomicBoolean(false);
    private volatile ServiceInstance serviceInstance;
    private volatile MetadataService metadataService;
    private volatile Set<MetadataServiceExporter> metadataServiceExporters;
    private List<ServiceConfigBase<?>> exportedServices = new ArrayList<>();
    private List<Future<?>> asyncExportingFutures = new ArrayList<>();
    private List<CompletableFuture<Object>> asyncReferringFutures = new ArrayList<>();
    /**
     * See {@link ApplicationModel} and {@link ExtensionLoader} for why DubboBootstrap is designed to be singleton.
     */
    public static DubboBootstrap getInstance() {
        if (instance == null) {
            synchronized (DubboBootstrap.class) {
                if (instance == null) {
                    instance = new DubboBootstrap();
                }
            }
        }
        return instance;
    }
    private DubboBootstrap() {
        configManager = ApplicationModel.getConfigManager();
        environment = ApplicationModel.getEnvironment();
        DubboShutdownHook.getDubboShutdownHook().register();