Cannot save video in run_mpc.py file

[robopassio]

I can run the demo of run_deep_highmpc as well as saving the animation perfectly.
However, I cannot save the animation with the file run_mpc.py after uncommenting the following lines:

    # run_frame = partial(run_mpc, env)
    # ani = animation.FuncAnimation(sim_visual.fig, sim_visual.update, frames=run_frame,
    #         init_func=sim_visual.init_animate, interval=100, blit=True, repeat=False)
    
    # #
    # if args.save_video:
    #     writer = animation.writers["ffmpeg"]
    #     writer = writer(fps=10, metadata=dict(artist='Me'), bitrate=1800)
    #     ani.save("MPC_0.mp4", writer=writer)
    
    # plt.tight_layout()
    # plt.show()

The following error happened:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-d4934ae49ad0> in <module>
     21 writer = animation.writers["ffmpeg"]
     22 writer = writer(fps=10, metadata=dict(artist='Me'), bitrate=1800)
---> 23 ani.save("MPC_0.mp4", writer=writer)
     24 
     25 plt.tight_layout()

/usr/local/lib/python3.6/dist-packages/matplotlib/animation.py in save(self, filename, writer, fps, dpi, codec, bitrate, extra_args, metadata, extra_anim, savefig_kwargs, progress_callback)
   1147                 else:
   1148                     total_frames = sum(save_count_list)
-> 1149                 for data in zip(*[a.new_saved_frame_seq() for a in all_anim]):
   1150                     for anim, d in zip(all_anim, data):
   1151                         # TODO: See if turning off blit is really necessary

/usr/local/lib/python3.6/dist-packages/matplotlib/animation.py in <listcomp>(.0)
   1147                 else:
   1148                     total_frames = sum(save_count_list)
-> 1149                 for data in zip(*[a.new_saved_frame_seq() for a in all_anim]):
   1150                     for anim, d in zip(all_anim, data):
   1151                         # TODO: See if turning off blit is really necessary

/usr/local/lib/python3.6/dist-packages/matplotlib/animation.py in new_saved_frame_seq(self)
   1704         else:
   1705             if self.save_count is not None:
-> 1706                 return itertools.islice(self.new_frame_seq(), self.save_count)
   1707 
   1708             else:

TypeError: 'NoneType' object is not iterable