In the Scene manager 1.1 demo moving up to the dark room works. But moving left to the next room gives the error : Attempt to call function 'get_parent' in base 'previously freed' on a null instance.
This occurs in the start_screen.gd file in the _on_level_added method on line 32 just after you check for null on the _loading_screen object.
I hope that helps.
I'm using Godot 4.3 beta 1