### Environment

* Dubbo version: 3.0
StaticDirectory.destroy方法最后要对列表做clear操作，但是ReferenceConfig的createInvokerForLocal构造StaticDirectory传入的列表类型是Collections.singletonList，执行clear就会抛异常：
```
[08/10/21 07:35:39:302 UTC] main  WARN config.ReferenceConfig:  [DUBBO] Unexpected error occurred when destroy invoker of ReferenceConfig(null)., dubbo version: 3.0.4-SNAPSHOT, current host: 172.19.112.1
java.lang.UnsupportedOperationException
	at java.util.AbstractList.remove(AbstractList.java:161)
	at java.util.AbstractList$Itr.remove(AbstractList.java:374)
	at java.util.AbstractList.removeRange(AbstractList.java:571)
	at java.util.AbstractList.clear(AbstractList.java:234)
	at org.apache.dubbo.rpc.cluster.directory.StaticDirectory.destroy(StaticDirectory.java:91)
	at org.apache.dubbo.rpc.cluster.support.AbstractClusterInvoker.destroy(AbstractClusterInvoker.java:113)
	at org.apache.dubbo.rpc.cluster.support.wrapper.MockClusterInvoker.destroy(MockClusterInvoker.java:79)
	at org.apache.dubbo.config.ReferenceConfig.destroy(ReferenceConfig.java:219)
```
urls.add(u.putAttribute(REFER_KEY, referenceParameters));
            }
        }
        if (urls.isEmpty()) {
            throw new IllegalStateException(
                "No such any registry to reference " + interfaceName + " on the consumer " + NetUtils.getLocalHost() +
                    " use dubbo version " + Version.getVersion() +
                    ", please config <dubbo:registry address=\"...\" /> to your spring config.");
        }
    }
    /**
     * Make a remote reference, create a remote reference invoker
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    private void createInvokerForRemote() {
        if (urls.size() == 1) {
            URL curUrl = urls.get(0);
            invoker = protocolSPI.refer(interfaceClass,curUrl);
            if (!UrlUtils.isRegistry(curUrl)){
                List<Invoker<?>> invokers = new ArrayList<>();
                invokers.add(invoker);
                invoker = Cluster.getCluster(scopeModel, Cluster.DEFAULT).join(new StaticDirectory(curUrl, invokers), true);
            }
        } else {
            List<Invoker<?>> invokers = new ArrayList<>();
            URL registryUrl = null;
            for (URL url : urls) {
                // 
                // 
                invokers.add(protocolSPI.refer(interfaceClass, url));