Hello gracious developer,
I am a fan of your tutorials and I plan on using this Scene Manager to properly develop some games.
I'm unsure of the best way to contribute to your source code as I ran into an issue. It might be because I am using Godot v4.3 beta2.
Running your example game, when I run through the door immediately to the left I error out with "Attempt to call function 'get_parent_ in base 'previously freed' on a null instance" (gameplay.gd:32 - at function: _on_level_added).
The issue is that when the loading screen is freed the variable _loading_screen becomes "<Freed Object>" and not null. _loading_screen != null is satisfied and that block of code runs even though _loading_screen is intended to be null.
Solution: replace line 31 with `if is_instance_valid(_loading_screen):`
When you run that you will get a similar error again somewhere else and then just repeat the solution.
I apologize for the essay, but I hope this helps someone.