Binding problems by using a RadGridView in a RadComboBox

[Michael]

Hello,

i am using a RadGridview with sample data in a combobox popup.

<Popup x:Name="PART_Popup" Margin="0,1,0,0">
    <Border BorderBrush="#FF595959" BorderThickness="1" Background="#FF3D3D3D"
         MaxHeight="{TemplateBinding MaxDropDownHeight}"
         MinWidth="{TemplateBinding MinDropDownWidth}">
         <telerik:RadGridView x:Name="RadGridView1"
              ShowGroupPanel="False" CanUserFreezeColumns="False"
              RowIndicatorVisibility="Collapsed" IsReadOnly="True"
              IsFilteringAllowed="True" ItemsSource="{Binding Dokumente}"
              MaxWidth="500" MaxHeight="400"
              SelectedItem="{Binding SelectedItem, Mode=TwoWay}">
         </telerik:RadGridView>
    </Border>
</Popup>

The ItemSource of my RadGridView is a sample data element which fills the RadGridView with data. RadGridViewInComboBox

My problem is, that i can not bind the value of an selected Cell/Row to the ComboBox text.

In the example from Vladimir Enchev, the binding works, but the data of the GridView is not from sample data.
http://blogs.telerik.com/vladimirenchev/posts/10-04-09/how_to_filter_as_you_type_radgridview_inside_radcombobox_for_wpf_and_silverlight.aspx

Can you give me a solution for my Problem?

Greetings Michael