### 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]

this.addTargetDataListener(path, treeCacheListener, null);
    }
    @Override
    protected void addTargetDataListener(String path, CuratorZookeeperClient.CuratorWatcherImpl treeCacheListener, Executor executor) {
        try {
            TreeCache treeCache = TreeCache.newBuilder(client, path).setCacheData(false).build();
            treeCacheMap.putIfAbsent(path, treeCache);
            treeCache.start();
            if (executor == null) {
                treeCache.getListenable().addListener(treeCacheListener);
            } else {
                treeCache.getListenable().addListener(treeCacheListener, executor);
            }
        } catch (Exception e) {
            throw new IllegalStateException("Add treeCache listener for path:" + path, e);
        }
    }
    @Override
    protected void removeTargetDataListener(String path, CuratorZookeeperClient.CuratorWatcherImpl treeCacheListener) {
        TreeCache treeCache = treeCacheMap.get(path);
        if (treeCache != null) {
            treeCache.getListenable().removeListener(treeCacheListener);
        }
        treeCacheListener.dataListener = null;
    }
    @Override
    public void removeTargetChildListener(String path, CuratorWatcherImpl listener) {
        listener.unwatch();
    }
    static class CuratorWatcherImpl implements CuratorWatcher, TreeCacheListener {