Code works in Editor but not in standalone

void GenerateBlankMap(int mSize)
mapSize = mSize;

        //Clear any children that may exist
        for (int i = 0; i < parent.childCount; i++)

        map = new List<List<Tile>>();

        for (int x = 0; x < mapSize; x++)
            List<Tile> row = new List<Tile>();

            for (int y = 0; y < mapSize; y++)
                //Create a tile set its position,
                //Add it to the row list,
                //Set tiles parent to be this gameobject
                Tile tile = ((GameObject)Instantiate(PrefabHolder.instance.TILE_BASE_PREFAB,
                                                     new Vector3(x - Mathf.Floor(mapSize / 2), 0,
                                                                -y + Mathf.Floor(mapSize / 2)),
                                                                Quaternion.Euler(new Vector3()))).
                tile.gridPosition = new Vector2(x, y);

This code works fine in the editor but when I build it out for standalone use its gives a me a null reference exception when it reaches the Instaniate.
Any help on solving this problem would be greatly appreciated.

Solved the problem. Just called the GenerateBlankMap function in the Start function instead of Awake.