Flappy Bird 2D，NullReferenceException，Unity

Question

我在Unity自己的教程的帮助下制作Flappy Bird，并设法一直到视频，在那里他展示了如何添加游戏控制器。一切都很好，但我似乎无法弄清楚如何摆脱这个错误：

  

NullReferenceException：未将对象引用设置为对象的实例   Bird.OnCollisionEnter2D（）（在Assets / Scripts / Bird.cs：37）

这是GameController的代码：

using UnityEngine;
using System.Collections;

public class GameControl : MonoBehaviour 
{

    public static GameControl instance;
    public GameObject gameOverText;
    public bool gameOver = false;

    // Use this for initialization
    void Awake () 
    {
        if (instance == null) {
            instance = this;
        }  
        else if (instance != this)
        {
            Destroy (gameObject);
        }
    }

    // Update is called once per frame
    void Update () {

    }

    public void BirdDied()
    {
        gameOverText.SetActive (true);
        gameOver = true;
    }
}

这是Bird.cs：

using UnityEngine;
using System.Collections;

 public class Bird : MonoBehaviour {

public float upForce = 200f;

private bool isDead = false;
private Rigidbody2D rb2d;
private Animator anim;


void Start ()
{
    rb2d = GetComponent<Rigidbody2D> ();
    anim = GetComponent<Animator> ();
}


void Update ()
{
    if (isDead == false)
    {
        if (Input.GetMouseButtonDown (0))
        {
            rb2d.velocity = Vector2.zero;
            rb2d.AddForce (new Vector2 (0, upForce));
            anim.SetTrigger ("Flap");
        }
    }
}

void OnCollisionEnter2D ()
{
    isDead = true;
    anim.SetTrigger ("Die");
    GameControl.instance.BirdDied ();
}
}

我设法让它工作，我不确定我到底做了什么，但我在代码中没有改变任何内容。