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++)
         {
             Destroy(parent.GetChild(i).gameObject);
         }
 
         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()))).
                                                                 GetComponent<Tile>();
                 tile.gridPosition = new Vector2(x, y);
                 tile.SetType(TileType.Grass);
                 row.Add(tile);
                 tile.transform.SetParent(parent);
             }
             map.Add(row);
         }
     }

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.