Index: lams_tool_scratchie/src/java/org/lamsfoundation/lams/tool/scratchie/web/controller/LearningWebsocketServer.java
===================================================================
diff -u -r18a207719c2aa30f683987fd0bb9176521b704de -rb8797193dabd790bbeb45f08fbbae52f0f07bf6d
--- lams_tool_scratchie/src/java/org/lamsfoundation/lams/tool/scratchie/web/controller/LearningWebsocketServer.java	(.../LearningWebsocketServer.java)	(revision 18a207719c2aa30f683987fd0bb9176521b704de)
+++ lams_tool_scratchie/src/java/org/lamsfoundation/lams/tool/scratchie/web/controller/LearningWebsocketServer.java	(.../LearningWebsocketServer.java)	(revision b8797193dabd790bbeb45f08fbbae52f0f07bf6d)
@@ -56,23 +56,24 @@
 		try {
 		    // websocket communication bypasses standard HTTP filters, so Hibernate session needs to be initialised manually
 		    HibernateSessionManager.openSession();
-		    
+
 		    Iterator<Entry<Long, Set<Session>>> entryIterator = LearningWebsocketServer.websockets.entrySet()
 			    .iterator();
 		    // go through activities and update registered learners with reports and vote count
 		    while (entryIterator.hasNext()) {
 			Entry<Long, Set<Session>> entry = entryIterator.next();
 			Long toolSessionId = entry.getKey();
-			// if all learners left the activity, remove the obsolete mapping
+
 			Set<Session> sessionWebsockets = entry.getValue();
-			if (sessionWebsockets.isEmpty()) {
+			ScratchieSession toolSession = LearningWebsocketServer.getScratchieService()
+				.getScratchieSessionBySessionId(toolSessionId);
+			// if all learners left the activity or session is missing, remove the obsolete mapping
+			if (sessionWebsockets.isEmpty() || toolSession == null) {
 			    entryIterator.remove();
 			    LearningWebsocketServer.cache.remove(toolSessionId);
 			    continue;
 			}
 
-			ScratchieSession toolSession = LearningWebsocketServer.getScratchieService()
-				.getScratchieSessionBySessionId(toolSessionId);
 			boolean timeLimitUp = false;
 			boolean scratchingFinished = toolSession.isScratchingFinished();
 			// is Scratchie time limited?
@@ -126,6 +127,8 @@
 		} catch (IllegalStateException e) {
 		    // do nothing as server is probably shutting down and we could not obtain Hibernate session
 		} catch (Exception e) {
+		    //TODO remove this once NullPointerExceptions do not show anymore in logs
+		    e.printStackTrace();
 		    // error caught, but carry on
 		    log.error("Error in Scratchie worker thread", e);
 		} finally {