Clear contents of GridDropDownColumn with EntityDataSource?

[Peter]

How do I clear the contents of a GridDropDownColumn (RadComboBox) which is bound to a EntityDataSource? 

The following code changes the data source of a cascading drop-down, but doesn't clear the contents of the RadComboBox - possibly because it's bound to the the EntityDataSource... (I hope to avoid doing a UNION NULL on the EntityDataSource).

protected void rgUnits_ItemCreated(object sender, GridItemEventArgs e)
       {
           GridEditableItem item = e.Item as GridEditableItem;

           if (item != null &&
               item.IsInEditMode &&
               item.OwnerTableView.DataSourceID != DataSourceIDUnitItem) // Parent Row
           {
               RadComboBox combo = (RadComboBox)item["EquipmentType"].Controls[0];
               combo.AllowCustomText = true;
               combo.AutoPostBack = true;
               combo.SelectedIndexChanged += combo_SelectedIndexChanged;
           }
       }

       void combo_SelectedIndexChanged(object sender, RadComboBoxSelectedIndexChangedEventArgs e)
       {
           GridEditableItem editedItem = (sender as RadComboBox).NamingContainer as GridEditableItem;

           int equipmentTypeID = 0;
           if (int.TryParse(((RadComboBox)editedItem["EquipmentType"].Controls[0]).SelectedValue, out equipmentTypeID))
           {
               EquipmentDescriptionDataSource.Where = string.Format(EquipmentDescriptionFilter, equipmentTypeID);

               RadComboBox rcbEquipmentDescription = ((RadComboBox)editedItem["EquipmentDescription"].Controls[0]);
               rcbEquipmentDescription.DataBind();
               rcbEquipmentDescription.ClearSelection();
               rcbEquipmentDescription.Text = string.Empty;
           }
       }

Any thoughts?

[Peter]

I worked it out, the key settings on the GridDropDownColumn are:

EnableEmptyListItem="true" ConvertEmptyStringToNull="true"