private void LoadComboBox()
        {
            GroupByList.Clear();
            GroupThenList.Clear();

            foreach (GroupingMode mode in Enum.GetValues(typeof(GroupingMode)).Cast <GroupingMode>())
            {
                GroupThenList.Add(mode);
                GroupByList.Add(mode);
            }

            if (Application.MainDocument.Grids.ActiveSystem == null)
            {
                GroupByList.Remove(GroupingMode.GridIntersection);
                GroupByList.Remove(GroupingMode.Level);
                GroupThenList.Remove(GroupingMode.GridIntersection);
                GroupThenList.Remove(GroupingMode.Level);
            }

            comboBoxGroupBy.SelectedIndex = 0;
            comboBoxThenBy.SelectedIndex  = 0;
        }