Ok, never used the shorthand, or even looked for one which is why it stuck out to me I guess.
edit: the error looks to be coming from the update_contacts() in base_char.gd
Does the error only occur if there is nothing to slide on perhaps?
func update_contacts() -> void:
if get_slide_count() <= 0:
return
If so then this bit might be the cause. I don't remember which issue but I seem to recall someone having some issue with physics where they added a return value to fix their issue. This is pretty much as good as a blind guess on my part though. Haven't dealt with physics much personally(more of a graphics/shaders guy myself).