### 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
for (Invoker<T> invoker : new ArrayList<>(localUrlInvokerMap.values())) {
                if (invoker.isAvailable()) {
                    return true;
                }
            }
        }
        return false;
    }
    public void buildRouterChain(URL url) {
        this.setRouterChain(RouterChain.buildChain(url));
    }
    /**
     * Haomin: added for test purpose
     */
    public Map<String, Invoker<T>> getUrlInvokerMap() {
        return urlInvokerMap;
    }
    public List<Invoker<T>> getInvokers() {
        return invokers;
    }
    private boolean isValidCategory(URL url) {
        String category = url.getParameter(CATEGORY_KEY, DEFAULT_CATEGORY);
        if ((ROUTERS_CATEGORY.equals(category) || ROUTE_PROTOCOL.equals(url.getProtocol())) ||
                PROVIDERS_CATEGORY.equals(category) ||
                CONFIGURATORS_CATEGORY.equals(category) || DYNAMIC_CONFIGURATORS_CATEGORY.equals(category) ||
                APP_DYNAMIC_CONFIGURATORS_CATEGORY.equals(category)) {
            return true;
        }
        logger.warn("Unsupported category " + category + " in notified url: " + url + " from registry " +
                getUrl().getAddress() + " to consumer " + NetUtils.getLocalHost());