Maybe try this, so the node path is relative:
get_parent().get_parent().get_node("AnimationPlayer").play("HideExit")
Then if the scene is instanced or something like that, it should work.
You can also use an exported NodePath if you want, so then you can set the AnimationPlayer through the Godot editor:
export (NodePath) var path_to_anim
_on_ExtraButton_pressed():
get_node(path_to_anim).play("HideExit")
Then to set up the NodePath you just need to select the node in the Godot editor and go to the inpsector, then there should be a property for the NodePath. :smile: