[Dubbo-4115] When the network is reconnected, the listener should not to be empty. (#4116)

Author: zonghaishang

dubbo-remoting/dubbo-remoting-etcd3/src/main/java/org/apache/dubbo/remoting/etcd/jetcd/JEtcdClient.java

@@ -278,7 +278,14 @@ public void unwatch() {
             }
 
             try {
-                this.listener = null;
+                /**
+                 * issue : https://github.com/apache/incubator-dubbo/issues/4115
+                 *
+                 * When the network is reconnected, the listener is empty
+                 * and the data cannot be received.
+                 */
+                // this.listener = null;
+
                 if (watchRequest != null) {
                     WatchCancelRequest watchCancelRequest =
                             WatchCancelRequest.newBuilder().setWatchId(watchId).build();