Also this one which he has a second script he uses to load and unload scenes:
One thing about loading and unloading proxies is that you have to use the same object that loaded them the first time. This is why we use a a separate script called loader or controller usually, that we use to load and unload all the scenes. When you try and load and unload a scene from itself youll usually get a “scene already loaded” error or some such.