Space Shooter 第七節中子彈生成部分提示Error

MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it.

嘗試呼叫的專案GameObject已被銷燬

具體表現為當螢幕中所有子彈都因越過邊界被銷燬時，應用於子彈生成的GameObject卻顯示為bolt(clone)(clone)(clone)(clone)，專案不存在導致Error。

但按照官方教程的嚴謹性應該不會出現這樣低階的錯誤。

解決方法如下（僅修改第18，28，29行處即可）：

using UnityEngine;
using System.Collections;

[System.Serializable]
public class Boundary
{
	public float xMin, xMax, zMin, zMax;
}


public class PlayerController : MonoBehaviour 
{

	public float speed;
	public float tilt;
	public Boundary boundary;

	public GameObject bolt;
	public GameObject shotSpawn; 
	public float fireRate;

	private float nextFire; 

	void Update () 
	{
		if (Input.GetKey("space") && Time.time > nextFire && shotSpawn != null) 
		{
			GameObject shot = Instantiate(bolt) as GameObject;
			shot.name = bolt.name;
			nextFire = Time.time + fireRate;
			shot.transform.position = shotSpawn.transform.position;

		}
	}
	void FixedUpdate ()
	{

		float moveHorizontal = Input.GetAxis ("Horizontal");
		float moveVertical = Input.GetAxis ("Vertical");

		Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
		GetComponent<Rigidbody>().velocity = movement * speed;

		GetComponent<Rigidbody>().position = new Vector3
			(
				Mathf.Clamp (GetComponent<Rigidbody>().position.x, boundary.xMin, boundary.xMax), 0.0f, Mathf.Clamp (GetComponent<Rigidbody>().position.z, boundary.zMin, boundary.zMax)
			);
		GetComponent<Rigidbody>().rotation = Quaternion.Euler (0.0f, 0.0f, GetComponent<Rigidbody>().velocity.x * -tilt);
	}
}