Thank you Veronica,
It is right; all my items are server side dynamically created; the same technique works without problems for a not combobox embedded treeview; like this:
<telerik:RadComboBox id = "_Employee_ComboBox_"
EmptyMessage = "Choose an employee..."
runat = "server">
comboBoxItem As RadComboBoxItem
For Each employee As Employee In employees
comboBoxItem.Text = employee.LastName &
", " & employee.FirstName
So, what I think the code flow of the combobox embedded treeview *forgets* (leaves undefined) about the parent/child widgets relationship; this is a bug. Debugging I see that the items collection of the combobox is empty.