### Environment

* Dubbo version: 2.7.6-SNAPSHOT
* Operating System version: Mac
* Java version: 1.8

### Steps to reproduce this issue

run dubbo-samples-simplified-registry-xml 

org.apache.dubbo.samples.simplified.registry.xml.DemoServiceIT
### Expected Result

should pass

### Actual Result

throw exception
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 (EMPTY_PROTOCOL.equals(url.getProtocol())) {
                continue;
            }
            String routerType = url.getParameter(ROUTER_KEY);
            if (routerType != null && routerType.length() > 0) {
                url = url.setProtocol(routerType);
            }
            try {
                Router router = ROUTER_FACTORY.getRouter(url);
                if (!routers.contains(router)) {