so im trying to code a bullet script using a rigid body 3D and its going good, but ive hit a wall and i cant figure out how to get past it

godot keeps on saying that the index "rotation" is invalid and on a null instance, idk what to do (im using 3D btw)

heres my code: 

extends RigidBody3D

func _physics_process(delta):

var impulse_strength = 100

var angle = $RigidBody3D.direction3D

$RigidBody3D.apply_central_impulse(Vector2(cos(angle), sin(angle)) * impulse_strength)