When using:

Value = "(null)"
Text = "(no user)"

as item in the ListBox, the filter must work correctly, and after opening the menu the item should be checked if previously filtered.

Additionally if the item have NULL value instead of "(null)" The visible value must be localized. 

The error is observed when an item that is added from the client-side is to be transferred to other RadListBox from a RadListBox that is loading on demand. Unfortunately the control supports such scenario only when that transfer is made from the server side.

Now, in WCF binding we return RadListBoxItemData collection which is not consistent with RadComboBox and RadDropDownList controls where we use RadControlNameData object.