First of all, and most important. The game_manager.gd is loaded as an Autoload, which shouldn’t be for this use case. Remove the GameManager autoload by pressing the trash can at the right of this settings.
Since GameManager is an auto load you don’t need to cache the reference.
It just
GameManager.do_a_func()
Autoloads are also siblings of the main scene it should be
$"../Main/UI/Panel/PointsLabel"
I would also advise against a reference like that from an auto load. The game manager should provide a signal that the UI panel will connect itself to.
In Level2 you have exact same error you had in Level1, the script attached to GameManager It’s not game_manager.gd. Remove that current script, and use game_manager.gd instead.
Once I did that, the game stopped crashing for me.