about RadTreeView's IsInEditMode property

[Bill]

Hi telerik:
     View:

<Window.Resources>

        <HierarchicalDataTemplate x:Key="TreeItemTemplate" ItemsSource="{Binding Children}">

            <TextBlock Text="{Binding Header}" />

        </HierarchicalDataTemplate>

        <Style TargetType="telerik:RadTreeViewItem">

            <Setter Property="IsExpanded" Value="true" />

            <Setter Property="IsSelected" Value="{Binding Selected, Mode=TwoWay}" />

            <Setter Property="IsExpanded" Value="{Binding Expanded, Mode=TwoWay}" />

            <Setter Property="IsEditable" Value="{Binding IsEditable, Mode=TwoWay}"/>

            <Setter Property="IsInEditMode" Value="{Binding IsInEditMode, Mode=TwoWay}"/>

        </Style>

    </Window.Resources>

    <Grid>

        <StackPanel>

            <Button Content="Add" Margin="0,20" Width="40" HorizontalAlignment="Left" Click="Button_Click"></Button>

        <telerik:RadTreeView Grid.Row="1" IsEditable="True"

                telerik:AnimationManager.IsAnimationEnabled="False" ItemsSource="{Binding Items}"

                ItemTemplate="{StaticResource TreeItemTemplate}"

                             ScrollViewer.VerticalScrollBarVisibility="Auto"/>

        </StackPanel>

    </Grid>


When I Add a Item,set the IsInEditMode=true,click the Add button continuously,the last time added item,the Name will be null

why?