我得到“对象示例未设置为示例对象”

private Dictionary<Vector3, GameObject> wallPositions;

    void Start()
    {
        size = 6;
        HalfMaze = new GameObject("Half Maze");
        traversed = new bool[size, size, size];
        Dictionary<Vector3, GameObject> wallPositions = new Dictionary<Vector3, GameObject>();
        GenerateGrid();
    }

//in some function
        foreach (Transform child in HalfMaze.transform) {
            //Debug.Log("hi2");
            foreach (Transform grandchild in child) {
                //Debug.Log(grandchild.gameObject.name);
                if (!(grandchild.position.x == -0.5f || grandchild.position.x == -0.5f + size || grandchild.position.y == -0.5f || grandchild.position.y == -0.5f + size || grandchild.position.z == 0 || grandchild.position.z == -0.5f + size)) {
                    wallPositions[grandchild.position] = grandchild.gameObject; //issue is here
                    //Debug.Log("hi");
                }
            }
        }

第二个代码块中的if语句内部导致运行时出现问题。我不知道为什么我得到的问题，因为孙子。gameObject不为null。