void OnCollisionEnter(Collision collision)
 {
     if (collision.gameObject.tag == "Player01")
     {
         // ... stop the nav mesh agent
         nav.Stop();
         // ... set the PlayerDead parameter inside Animator Controller
         animator.SetTrigger("PlayerDead");
         // ... call players Death function
         Player_controller playerController = player.gameObject.GetComponent <Player_controller>();
         playerController.Death();
     }
 }