don't repaint fields after game end

src/BoardDisplay.java

@@ -174,6 +174,7 @@ public class BoardDisplay extends JPanel {
         if(currentShip != null) {
             test = currentShip.getVirtualOccupiedPoints(mousePosition, horizontal);
         }
+        if (player == null || player.getBoard() == null) return;
         for(int i = 0; i < gridSize; i++) {
             for(int j = 0; j < gridSize; j++) {
                 if(fields[i][j] == null) {