private void DataGrid_FilterItemsPopulated(object sender, Syncfusion.UI.Xaml.Grid.GridFilterItemsPopulatedEventArgs e)
{
    (e.ItemsSource as List<FilterElement>).Sort(new FilterElementComparer());
}


public class FilterElementComparer : IComparer<FilterElement>
{
    public int Compare(FilterElement x, FilterElement y)
    {
        if (x == null)
        {
            if (y == null)
            {
                // If x is null and y is null, they're 
                // equal. 
                return 0;
            }
            else
            {
                // If x is null and y is not null, y 
                // is greater. 
                return -1;
            }
        }
        else
        {
            // If x is not null... 
            // 
            if (y == null)
            // ...and y is null, x is greater. 
            {
                return 1;
            }
            else
            {
                return x.DisplayText.CompareTo(y.DisplayText);
            }
        }
    }
}