diff -r f7433671fec5 -r 1c62c6009161 src/main/java/de/uapcore/sudoku/ButtonPanel.java
--- a/src/main/java/de/uapcore/sudoku/ButtonPanel.java	Sat Jul 25 15:56:48 2020 +0200
+++ b/src/main/java/de/uapcore/sudoku/ButtonPanel.java	Mon Jul 27 10:56:17 2020 +0200
@@ -34,7 +34,7 @@
  */
 public final class ButtonPanel extends JPanel {
     
-    private JButton save, check, solve;
+    private final JButton save, check, solve;
 
     public ButtonPanel(ActionHandler l) {
         setLayout(new GridBagLayout());