GUI- poke the traffic monitor to resend highlight data, on topology event.
 - also, fix potential NPE (.el element may be null)

Change-Id: Ibdb5085404e81bfa770ee80d1457bc630635a169
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 f9063ad..f25ef91 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
@@ -741,6 +741,7 @@
         public void event(DeviceEvent event) {
             if (event.type() != PORT_STATS_UPDATED) {
                 msgSender.execute(() -> sendMessage(deviceMessage(event)));
+                msgSender.execute(traffic::pokeIntent);
                 eventAccummulator.add(event);
             }
         }
@@ -751,6 +752,7 @@
         @Override
         public void event(LinkEvent event) {
             msgSender.execute(() -> sendMessage(linkMessage(event)));
+            msgSender.execute(traffic::pokeIntent);
             eventAccummulator.add(event);
         }
     }
@@ -760,6 +762,7 @@
         @Override
         public void event(HostEvent event) {
             msgSender.execute(() -> sendMessage(hostMessage(event)));
+            msgSender.execute(traffic::pokeIntent);
             eventAccummulator.add(event);
         }
     }