No one of those below works! How to just disable a button?
get_tree().get_root().get_node("Control").LoadButton.disable = true
get_tree().get_root().get_node("Control").LoadButton.disable(true)
get_tree().get_root().get_node("Control").LoadButton.disable()
get_tree().get_root().get_node("Control").LoadButton.set_disable()