added error handling when requesting to join non existing room

src/Controller/GameController.php

@@ -114,6 +114,11 @@ class GameController
         }
 
         $room = $this->multiRoomRepository->getByRoomId($roomId);
+
+        if(!isset($room)) {
+            return new JsonContent(['error' => 'game_not_found']);
+        }
+
         $state = $room->getStateArray();
         $map = $this->mapRepository->getById($state['mapId']);
         $token = $this->getMultiToken($roomId);