### Environment

* Dubbo version: 2.71
* Operating System version: Windows 10
* Java version: Oracle JDK 1.8.1_191

### Steps to reproduce this issue
使用curator版本2.13.0，作为dubbo:refenence，应用启动报错如下异常：
2019-04-12 15:40:24.145 ERROR ZookeeperDynamicConfiguration-thread-1 TreeCache.handleException:760 []
java.lang.NullPointerException: null
	at java.lang.String.<init>(String.java:515) ~[?:1.8.0_191]
	at org.apache.dubbo.remoting.zookeeper.curator.CuratorZookeeperClient$CuratorWatcherImpl.childEvent(CuratorZookeeperClient.java:298) ~[dubbo-2.7.1.jar:2.7.1]
	at org.apache.curator.framework.recipes.cache.TreeCache$2.apply(TreeCache.java:741) [curator-recipes-2.13.0.jar:?]
	at org.apache.curator.framework.recipes.cache.TreeCache$2.apply(TreeCache.java:735) [curator-recipes-2.13.0.jar:?]
	at org.apache.curator.framework.listen.ListenerContainer$1.run(ListenerContainer.java:93) [curator-framework-2.13.0.jar:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_191]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_191]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]

if (childListener != null) {
                String path = event.getPath() == null ? "" : event.getPath();
                childListener.childChanged(path,
                        
                        
                        
                        StringUtils.isNotEmpty(path)
                                ? client.getChildren().usingWatcher(this).forPath(path)
                                : Collections.<String>emptyList());
            }
        }
        @Override
        public void childEvent(CuratorFramework client, TreeCacheEvent event) throws Exception {
            if (dataListener != null) {
                TreeCacheEvent.Type type = event.getType();
                EventType eventType = null;
                String content = null;
                String path = null;
                switch (type) {
                    case NODE_ADDED:
                        eventType = EventType.NodeCreated;
                        path = event.getData().getPath();
                        content = new String(event.getData().getData(), CHARSET);
                        break;
                    case NODE_UPDATED:
                        eventType = EventType.NodeDataChanged;
                        path = event.getData().getPath();
                        content = new String(event.getData().getData(), CHARSET);
                        break;
                    case NODE_REMOVED: