### 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]

*/
package org.apache.dubbo.remoting.zookeeper.curator;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.remoting.zookeeper.ChildListener;
import org.apache.dubbo.remoting.zookeeper.DataListener;
import org.apache.dubbo.remoting.zookeeper.EventType;
import org.apache.dubbo.remoting.zookeeper.StateListener;
import org.apache.dubbo.remoting.zookeeper.support.AbstractZookeeperClient;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.api.CuratorWatcher;
import org.apache.curator.framework.recipes.cache.TreeCache;
import org.apache.curator.framework.recipes.cache.TreeCacheEvent;
import org.apache.curator.framework.recipes.cache.TreeCacheListener;
import org.apache.curator.framework.state.ConnectionState;
import org.apache.curator.framework.state.ConnectionStateListener;
import org.apache.curator.retry.RetryNTimes;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException.NoNodeException;
import org.apache.zookeeper.KeeperException.NodeExistsException;
import org.apache.zookeeper.WatchedEvent;
import java.nio.charset.Charset;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Executor;
import static org.apache.dubbo.common.constants.CommonConstants.TIMEOUT_KEY;
public class CuratorZookeeperClient extends AbstractZookeeperClient<CuratorZookeeperClient.CuratorWatcherImpl, CuratorZookeeperClient.CuratorWatcherImpl> {