Ok, never mind: solved the issue: basically you need to add return scene at the end of func post_import(scene):
ie:
tool # needed so it runs in editor
extends EditorScenePostImport
func post_import(scene):
print("this is our " + str(scene) + " and now we do lot of stuff to it! :evil:")
return scene # remember to return the imported scene
edit: don't ask me why markdown break the last two rows of code