The GUI stores its own game status in a linked list, which is updated after every move the player makes. This game status may be not synchronized with the actual game status of the game, stored in Game.java class.
Resolution: Modify logic for GUI to store game status, it copies the game status from Game.java after each move.
https://github.com/glennsalter/cs3343-chess/commit/0ccccfc9879166374c4d3d944ed0f25981f58d7c