Fixing an NPE in GUI idle detect mechanism; ONOS-441

Change-Id: Id1edd74642915a266d9e935287b70cf5425c91f6

@@ -168,14 +168,15 @@ public class TopologyViewWebSocket
      */
     synchronized boolean isIdle() {
         boolean idle = (System.currentTimeMillis() - lastActive) > MAX_AGE_MS;
-        if (idle || !connection.isOpen()) {
+        if (idle || (connection != null && !connection.isOpen())) {
             return true;
-        }
+        } else if (connection != null) {
             try {
                 control.sendControl(PING, PING_DATA, 0, PING_DATA.length);
             } catch (IOException e) {
                 log.warn("Unable to send ping message due to: ", e);
             }
+        }
         return false;
     }