When Godot tells you
in base "null instance", it means the object on which you call the function is
object_colliding is null.
One reason could be that
get_node("RayCast2D").get_collider() did not return any collider, which means there is no collision. So you should check for that case before checking the group of the collider.
if object_colliding != null && object_colliding.is_in_group("interractable_objects")