GUI -- Adding synchronization for message sending and for create/destroy handlers.

Change-Id: Id0cb367dcdaf658d3a58ef3f81718a08d1fd1438
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java b/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
index 854ac5a..9beeb94 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
@@ -589,7 +589,7 @@
 
 
     // Adds all internal listeners.
-    private void addListeners() {
+    private synchronized void addListeners() {
         listenersRemoved = false;
         clusterService.addListener(clusterListener);
         mastershipService.addListener(mastershipListener);