Add missing exception handler

app/socketio_events.py

@@ -33,7 +33,10 @@ def socketio_disconnect():
     ' On disconnect the session id gets removed from the connected sessions
     ' list.
     '''
-    socketio_sessions.remove(request.sid)
+    try:
+        socketio_sessions.remove(request.sid)
+    except ValueError:
+        pass
 
 
 @socketio.on('start_user_session')