Hello~
I also dodged to using the custom spawning of the Multiplayer spawner because I ran into issues with the add_child() approach. Some googling (and finally this solution by @camjknight HERE 
) led me down into using the custom spawn, but that also clicks better with my brain for some reason, to be honest.
Anyways, @AntiVirusJ I found that passing a parameter with spawn() solved the errors for me.
You don’t have to actually use it and can hide it behind an _underscore
Example:
func _tree_enter()
spawner.spawn_function = spawn_thing
func spawn_thing(_x):
return thing_scene.instantiate()
func _on_interaction():
spawner.spawn(1)