Your code is set up different to mine but try in the startGame() method to add setCenter(grid) after calling the setUpLabels() and your addButton() method (which btw I don't see called anywhere in your code.) I called my equivalent method in the setUpGame() method before g = new Game() 

Id move your Button objects out of the method so they live outside the scope of the method, and just call it like setuplabels() is called  In the start game method.