So, the question was already posed and answered, but this approach won't work for me:
var FancyTSButton = preload("res://addons/FancyTSButton/FancyTSButton.gd")
var button = FancyTSButton.new()
After running it, nothing at all happens. The only error it reports is
!is_inside_tree() is true while pointing to line 3. Any ideas?