Add width and height check for MujocoRenderer (#1230)

gymnasium/envs/mujoco/mujoco_rendering.py

@@ -702,6 +702,10 @@ def render(
         Returns:
             If render_mode is "rgb_array" or "depth_array" it returns a numpy array in the specified format. "human" render mode does not return anything.
         """
+        if render_mode != "human":
+            assert (
+                self.width is not None and self.height is not None
+            ), f"The width: {self.width} and height: {self.height} cannot be `None` when the render_mode is not `human`."
 
         viewer = self._get_viewer(render_mode=render_mode)