Well first off
get_collider() doesn't take a parameter so that code is just wrong.
It is failing because at the time you are calling the
get_collider() function (if it even works with being called with
"/root/Proto/Player" as the parameter) the engine can't find a child node of the node that this script is attached to named
null is just a fancy way of saying nothing.
null has no function named
get_collider() so it crashes.
That is absolutely awful code so I highly recommend following another tutorial. Check out the Youtube channels: GDQuest, KidsCanCode, Heartbeast, or Game Endeavor to learn how to properly create games in Godot.