void Update()
    {
        stayTime += Time.deltaTime;
        AB        = GameObject.FindObjectOfType <AutoBullet>();
        if (stayTime >= 20)
        {
            Destroy(this.gameObject);
        }

        if (AB == null)
        {
            Destroy(this.gameObject);
        }
    }
 void Start()
 {
     target = GameObject.FindGameObjectWithTag("Player").transform;
     AB     = GameObject.FindObjectOfType <AutoBullet>();
 }