Update tetris_env.py

tetris_env.py

@@ -91,6 +91,8 @@ class TetrisEnv(gym.Env):
         return self._get_obs(), reward, done, {}
 
     def _get_obs(self):
+        if self.game.field is None:
+            raise ValueError("The field attribute in self.game is None.")
         """
         Get the current state of the game as an observation
         """