Fix bug in simulate where pressing '[' or ']' when a model is not loaded will causes a crash.
Fix bug in simulate where pressing '[' or ']' when a model is not loaded will causes a crash.

PiperOrigin-RevId: 436791755
Change-Id: I0bfec45eecef4bbb8db8f47f0d1e8a0276399b8f

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco