So the log says
get_node fails at line 25 of
main.gd. Assuming you copied the same contents, it would be
mob_instance.set_path(path)... except there is no
get_node there, so I assume lines don't match. Instead it would be one of the
$ node accesses in this line, am I right?:
var path = $nav.get_simple_path($start_position.position, $end_position.position)
It means either
end_position is not a valid path. I can't really tell much otherwise.
I noticed though, that you used
instance() to create an new instance of the mob, but you are not adding it to the tree (at least in the code you posted) so it won't appear.
onready var mob = preload("res://mob.tscn") .instance()
That would not solve your problem, it actually makes it more confusing. If you want to instance several mobs, don't do it here, do it when you actually spawn them.