### 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]

static final Charset CHARSET = Charset.forName("UTF-8");
    private final CuratorFramework client;
    private Map<String, TreeCache> treeCacheMap = new ConcurrentHashMap<>();
    public CuratorZookeeperClient(URL url) {
        super(url);
        try {
            int timeout = url.getParameter(TIMEOUT_KEY, 5000);
            CuratorFrameworkFactory.Builder builder = CuratorFrameworkFactory.builder()
                    .connectString(url.getBackupAddress())
                    .retryPolicy(new RetryNTimes(1, 1000))
                    .connectionTimeoutMs(timeout);
            String authority = url.getAuthority();
            if (authority != null && authority.length() > 0) {
                builder = builder.authorization("digest", authority.getBytes());
            }
            client = builder.build();
            client.getConnectionStateListenable().addListener(new ConnectionStateListener() {
                @Override
                public void stateChanged(CuratorFramework client, ConnectionState state) {
                    if (state == ConnectionState.LOST) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.DISCONNECTED);
                    } else if (state == ConnectionState.CONNECTED) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.CONNECTED);
                    } else if (state == ConnectionState.RECONNECTED) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.RECONNECTED);
                    }
                }
            });
            client.start();
        } catch (Exception e) {