Excel-Like filtering how to uncheck checkbox values while opening the popup

private void RadGridView1_FilterPopupRequired(object sender, Telerik.WinControls.UI.FilterPopupRequiredEventArgs e)
{
// string column
    if (e.Column.Name == "Name")
    {
        this.radGridView1.Columns["Name"].FilterDescriptor = null;
        e.FilterPopup = new RadListFilterPopup(e.Column);
    }
}