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在做相机视野跟随的时候， 在运行游戏的时候，出现了异常，并且相机不跟随角色移动了。

异常信息

NullReferenceException: Object reference not set to an instance of an object
FollowTarget.Start () (at Assets/Scripts/FollowTarget.cs:13)
NullReferenceException: Object reference not set to an instance of an object
FollowTarget.Update () (at Assets/Scripts/FollowTarget.cs:18)

出问题的代码

	private Transform hero;
    void Start () {
        offset = transform.position - hero.position;// 13行
	}
	
	// Update is called once per frame
	void Update () {
        transform.position = offset + hero.position;// 18行
	}

unity 2017 ngui 3.6.8