For anyone experiencing the black box issue, maybe this will help:
Create a shortcut to the .exe. Go to the shortcut's settings and add this after the target path:
--rendering-driver opengl3
Use the shortcut to start the game.
Sadly, this did not work for me (my pet disappeared completely), but it helped for a different dektop pet made in Godot on a different computer! Maybe this works for someone else. Depends on the GPU, apparently (Nvidia mostly).