- [ ] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: xxx
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

1. xxx
2. xxx
3. xxx

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

What do you expected from the above steps？

### Actual Result

What actually happens?

If there is an exception, please attach the exception trace:

```
Just put your stack trace here!
```

for (List<Invoker<T>> groupList : groupMap.values()) {
                StaticDirectory<T> staticDirectory = new StaticDirectory<>(groupList);
                staticDirectory.buildRouterChain();
                mergedInvokers.add(cluster.join(staticDirectory));
            }
        } else {
            mergedInvokers = invokers;
        }
        return mergedInvokers;
    }
    /**
     * @param urls
     * @return null : no routers ,do nothing
     * else :routers list
     */
    private Optional<List<Router>> toRouters(List<URL> urls) {
        if (urls == null || urls.isEmpty()) {
            return Optional.empty();
        }
        List<Router> routers = new ArrayList<>();
        for (URL url : urls) {
            if (Constants.EMPTY_PROTOCOL.equals(url.getProtocol())) {
                continue;
            }
            String routerType = url.getParameter(Constants.ROUTER_KEY);
            if (routerType != null && routerType.length() > 0) {
                url = url.setProtocol(routerType);
            }
            try {
                Router router = routerFactory.getRouter(url);