Unity3D:列表从其他类访问时返回空值

我目前正在尝试在unity3D中制作一个小型手机游戏，并且由于我总是听到使用实例化/销毁是一种非常糟糕的方法(尽管我从未制作过足够大的东西，这实际上成为一个问题(我决定尝试在池系统中制作。

我创建了一个脚本来处理两个单独列表中的所有池对象。我可以通过从脚本本身调用方法成功地从池中生成一个对象，但问题是当我尝试从另一个脚本调用该方法时，它不起作用。通过我所做的不同尝试，我得到了空引用异常，或者得到了错误：

由于对象的当前状态，操作无效 System.Linq.Enumerable.First[GameObject] (IEnumerable'1 source(

我真的找不到其他人有这个问题。而且似乎相关的解决方案不起作用。主要问题也可能是..为什么会这样？

池脚本：

public class ObjectPool : MonoBehaviour {

    GameObject basicCell;
    GameObject basicFood;
    private Vector3 poolSpawnPos;
    [SerializeField]
    private int cellAmount;
    [SerializeField]
    private int foodAmount;
    List<GameObject> cellPool;
    List<GameObject> foodPool;
    GameObject foodManager;
    GameObject cellManager;
    // Use this for initialization
    void Awake () {
        foodManager = gameObject.transform.FindChild ("FoodPool").gameObject;
        cellManager = gameObject.transform.FindChild ("CellPool").gameObject;
        cellPool = new List<GameObject> (cellAmount);
        foodPool = new List<GameObject> (foodAmount);
        poolSpawnPos = new Vector3 (8000, 8000, 8000);
        basicFood = Resources.Load ("Food/BasicFood/BasicFood") as GameObject;
        basicCell = Resources.Load ("Cell/Cell") as GameObject;
        for (int i = 0; i < cellAmount; i++) {
            GameObject currentCell = Instantiate(basicCell, poolSpawnPos, Quaternion.identity) as GameObject;
            currentCell.transform.parent = cellManager.transform;
            currentCell.SetActive (false);
            cellPool.Add (currentCell);
        }
        for (int i = 0; i < foodAmount; i++){
            GameObject currentFood = Instantiate(basicFood, poolSpawnPos, Quaternion.identity) as GameObject;
            currentFood.transform.parent = foodManager.transform;
            currentFood.SetActive (false);
            foodPool.Add (currentFood);
        }
    }
    // Update is called once per frame
    void Update () {
        //if(foodPool.Count > 0)
        //  SpawnFood (new Vector3 (0, 0, 0));
    }
    public void SpawnFood(Vector3 spawnPosition)
    {
        GameObject selectedFood = null;
        selectedFood = foodPool.First();
        print (selectedFood);
        selectedFood.SetActive (true);
        foodPool.Remove (selectedFood);
        selectedFood.transform.parent = null;
        selectedFood.transform.position = spawnPosition;

        //set food Stats

    }

    public void KillFood(GameObject killedFood)
    {
        foodPool.Add (killedFood);
        killedFood.transform.position = poolSpawnPos;
        killedFood.transform.parent = foodManager.transform;
        killedFood.SetActive (false);
    }
}

我需要从中调用该方法的脚本：

public class FoodManager : MonoBehaviour {
    public ObjectPool pool;
    public int initialFoodNumber = 50;
    void Awake()
    {
        pool = GameObject.Find("ObjectPool").GetComponent<ObjectPool>();
        for (int i = 0; i <= initialFoodNumber; i++)
        {
            pool.SpawnFood(new Vector3(Random.Range(-40, 41), 0, Random.Range(-40, 41)));
        }
    }
}