### 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
* @param newUrlInvokerMap
     */
    private void destroyUnusedInvokers(Map<String, Invoker<T>> oldUrlInvokerMap, Map<String, Invoker<T>> newUrlInvokerMap) {
        if (newUrlInvokerMap == null || newUrlInvokerMap.size() == 0) {
            destroyAllInvokers();
            return;
        }
        
        List<String> deleted = null;
        if (oldUrlInvokerMap != null) {
            Collection<Invoker<T>> newInvokers = newUrlInvokerMap.values();
            for (Map.Entry<String, Invoker<T>> entry : oldUrlInvokerMap.entrySet()) {
                if (!newInvokers.contains(entry.getValue())) {
                    if (deleted == null) {
                        deleted = new ArrayList<>();
                    }
                    deleted.add(entry.getKey());
                }
            }
        }
        if (deleted != null) {
            for (String url : deleted) {
                if (url != null) {
                    Invoker<T> invoker = oldUrlInvokerMap.remove(url);
                    if (invoker != null) {
                        try {
                            invoker.destroy();
                            if (logger.isDebugEnabled()) {
                                logger.debug("destroy invoker[" + invoker.getUrl() + "] success. ");
                            }