raise ValueError instead of string

gym_minigrid/roomgrid.py

@@ -208,7 +208,8 @@ class RoomGrid(MiniGridEnv):
         elif kind == "box":
             obj = Box(color)
         else:
-            raise f"{kind} object kind is not available in this environment."
+            raise ValueError(
+                f"{kind} object kind is not available in this environment.")
 
         return self.place_in_room(i, j, obj)