bad custom fields should default to beginner values

[Namdrib]

in Minesweeper.rbMenuItem.addActionListener, entering a bad input for heightField, widthField or minesField (e.g. empty strings, non-numerical input, etc.) should result in beginner values for each incorrect field.

          try {
            newHeight = Integer.parseInt(heightField.getText());
          } catch (NumberFormatException ex) {
            newHeight = 9;
          }
          try {
            newWidth = Integer.parseInt(widthField.getText());
          } catch (NumberFormatException ex) {
            newWidth = 9;
          }
          try {
            newMines = Integer.parseInt(minesField.getText());
          } catch (NumberFormatException ex) {
            newMines = 10;
          }

This requires an individual try/catch since each field is treated individually

[Namdrib]

Current behaviour is to just abort changing the difficulty

[Namdrib]

Fixed by 9db1c0f