added a null check in RacingEventServiceImpl where the notificationService may be null, especially in test cases

Change-Id: I508b4fc8ae083b57ab49c1c0b809a5b10df33a5c

java/com.sap.sailing.server/src/com/sap/sailing/server/impl/RacingEventServiceImpl.java

@@ -2859,8 +2859,10 @@ public class RacingEventServiceImpl implements RacingEventService, ClearStateTes
         mediaLibrary.clear();
         competitorStore.clear();
         remoteSailingServerSet.clear();
-        notificationService.stop();
-        notificationService = new EmptySailingNotificationService();
+        if (notificationService != null) {
+            notificationService.stop();
+            notificationService = new EmptySailingNotificationService();
+        }
     }
 
     // Used for TESTING only