NullReferenceException: Object reference not set to an instance of an object

     public void EndSpell () {
         movement.setCanWalk (true);
         animator.SetInteger ("Spell", 0);
         GameObject projectile = (GameObject) Instantiate (selectedSpell.prefab, transform.position, Quaternion.Euler (new Vector3 (0, 0, dir))); //TODO ERROR
         Physics2D.IgnoreCollision (myCollider, projectile.GetComponent<Collider2D> ());
         Projectile_Movement movementScript = projectile.GetComponent<Projectile_Movement> ();
         movementScript.caster = gameObject;
         Vector3 v = new Vector3 (worldPoint.x - transform.position.x, worldPoint.y - transform.position.y, 0);
         movementScript.SetDestination (transform.position, v.normalized);
         selectedSpell = null;
         selectedSlot = -1;
         RefreshActionBarColors ();
     }

That is my code (do you need more of it?) and the error is in the line "//TODO ERROR". I have searched all over google but no solution for my case. Something is null but what? The error comes everytime when I cast a spell in my game but it still kinda works. Please someone help.. :)