@Override
    public @Nullable Object getValueAt(int rowIndex, int columnIndex) {
      T element = myElements.get(rowIndex);
      if (columnIndex == ELEMENT_COLUMN_INDEX) {
        return element;
      }
      if (columnIndex == CHECK_MARK_COLUM_INDEX) {
        return getElementMarkState(element);
      }
      return null;
    }
@Override
    public @Nullable Object getValueAt(int rowIndex, int columnIndex)