Invalid call.Nonexistent function'instance' in Base Nil. This is the problem i facing at the code below please help me

:information_source: Attention Topic was automatically imported from the old Question2Answer platform.
:bust_in_silhouette: Asked By hkw

extends KinematicBody2D

export(int)var detect_radius
export(float) var fire_rate
export(PackedScene)var Bullet
var vis_color = Color(.867,.91,.247)

var target
var can_shoot = true

func _ready():
$Sprite.self_modulate = Color(0.2,0,0)
var shape
shape.radius = detect_radius
$Visibility/CollisionShape2D.shape = shape
$ShootTimer.wait_time = fire_rate

func _physics_process(delta):
if target:
rotation = (target.position - position).angle()
if can_shoot:

func shoot(pos):
var b = Bullet.instance()
var a = (pos - global_position).angle()
b.start(global_position,a + rand_range(-0.05,0.05))
can_shoot = false

func _draw():

func _on_Visibility_body_entered(body):
if target:
target = body
$Sprite.self_modulate.r = 1.0

func _on_Visibility_body_exited(body):
if body == target:
target = null
$Sprite.self_modulate.r = 0.2

func _on_ShootTimer_timeout():
can_shoot = true

:bust_in_silhouette: Reply From: Wakatta

For future use this error means you have and invalid reference and your variable is not properly assigned.

In the following line you are expected to set the bullet PackedScene in the Inspector

export(PackedScene)var Bullet