NullReferenceException with 2d Array

Hello, I am trying to make a minesweeper game and need help with finding adjacent cells. I'm getting this error: "NullReferenceException: Object reference not set to an instance of an object" And cannot, for the life of me, find the source of the problem!

Board Class:

 public Tile[,] tiles;
 public Transform boardTransform;
 [SerializeField] private Vector2 boardSize;
 public int bombCount;
 public GameObject tileAsset;

 public static int boardSizeX;
 public static int boardSizeY;

 public static Board board;

 private void Awake ()
 {
     board = this;
 }

 private void Start ()
 {
     boardSizeX = Mathf.FloorToInt ( boardSize.x );
     boardSizeY = Mathf.FloorToInt ( boardSize.y );

     tiles = new Tile[boardSizeX, boardSizeY];
     CreateBoard ();
 }

 private void CreateBoard ()
 {
     if (boardTransform != null)
     {
         for (int x = 0; x < boardSizeX; x++)
         {
             for (int y = 0; y < boardSizeY; y++)
             {
                 Tile newTile = Instantiate ( tileAsset ).GetComponent<Tile> ();

                 newTile.transform.SetParent ( boardTransform );

                 tiles[x, y] = newTile;

                 newTile.xPos = x;
                 newTile.yPos = y;
             }
         }
         foreach (Tile tile in tiles)
         {
             tile.SetAdjacentTiles ();
         }
         PlaceBombs ();
     }
 }

Tile Class:

 public bool isOpen = false;
 public bool isBomb = false;
 public int xPos;
 public int yPos;

 private List<Tile> adjacentTiles;
 private Button _button;
 private Text _text;

 public int adjacentBombCount
 {
     get
     {
         int amount = 0;
         foreach (Tile tile in adjacentTiles)
         {
             if (tile.isBomb)
                 amount++;
         }
         return amount;
     }
 }

 private void Start ()
 {
     _button = GetComponent<Button> ();
     _text = GetComponentInChildren<Text> ();
     adjacentTiles = new List<Tile> ();
 }

 public void SetAdjacentTiles ()
 {
     for (int x = xPos - 1; x <= xPos + 1; x++)
     {
         if (x < 0 || x >= Board.boardSizeX)
             continue;

         for (int y = yPos - 1; y <= yPos + 1; y++)
         {
             if (y < 0 || y >= Board.boardSizeY)
                 continue;

             if (Board.board.tiles[x, y] != this)
                 adjacentTiles.Add ( Board.board.tiles[x, y] );        
         }
     }
 }

The error happens in "SetAdjacentTiles" at "adjacentTiles.Add ( Board.board.tiles[x, y] );"

Please help!

I'm a novice programmer so any advice would be much appreciated!

Thank you :)