在Tic Tac Toe GUI java + quit按钮中检测无效移动

I have my code almost entirely working and just ran into a couple of small problems. For one, I want the user to receive a popup when they try to select a button already marked with an X or an O, telling them the move is invalid and to pick another. Secondly, I'm trying to add a quit button to the bottom of the game, but it always makes the quit button part of the game grid? The solution is probably simple and I'm just not seeing it, but I've tried it a couple different times and can't seem to get it out of that grid. Thanks for all help.I have my code almost entirely working and just