Index: lams_learning/src/java/org/lamsfoundation/lams/learning/command/CommandWebsocketServer.java
===================================================================
diff -u -rd19a95db673b7cf90351de4d567299c5fa86b450 -r923f5bc207d13c2da27888f6595dbe8dd29e4b75
--- lams_learning/src/java/org/lamsfoundation/lams/learning/command/CommandWebsocketServer.java	(.../CommandWebsocketServer.java)	(revision d19a95db673b7cf90351de4d567299c5fa86b450)
+++ lams_learning/src/java/org/lamsfoundation/lams/learning/command/CommandWebsocketServer.java	(.../CommandWebsocketServer.java)	(revision 923f5bc207d13c2da27888f6595dbe8dd29e4b75)
@@ -149,9 +149,17 @@
      */
     @OnClose
     public void unregisterUser(Session session, CloseReason reason) {
+	String login = session.getUserPrincipal().getName();
+	if (login == null) {
+	    return;
+	}
+
 	Long lessonId = Long.valueOf(session.getRequestParameterMap().get(AttributeNames.PARAM_LESSON_ID).get(0));
 	Map<String, Session> lessonWebsockets = CommandWebsocketServer.websockets.get(lessonId);
-	String login = session.getUserPrincipal().getName();
+	if (lessonWebsockets == null) {
+	    return;
+	}
+
 	lessonWebsockets.remove(login);
     }