Treeview(desined as a usercontrol) popup inside a Button in WPF

Question

hi,

I would like to place a treewview usecontrol inside a button as a Popup. When I opened popup by clickin gbutton for first time, it works. I loaded some data depending on treeview selected time and closed popup. When I open the popup second time to choose another tree view item, it is not letting me to choose. Iam able to expand child nodes but not able to select any item including parent.

Here is my XAML code

   <Button   Name="mBtnBizUnitToolPopup" 
   Click="mBtnBizUnitToolPopup_Click" Focusable="False">
            <Grid
                <TextBlock 
     Name="mTxtSelectedBizUnit" 
     Grid.Row="0">{None Selected}</TextBlock>
               < Popup
     Grid.Row="1"
     Name="mBizUnitToolPopup"
     StaysOpen="False"
     IsOpen="False"
     Width="420"
     Height="600" Focusable="False">
                    <Grid Background="White">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="420" />
                            <RowDefinition Height="23" />
                        </Grid.RowDefinitions>
                        <local:UBizUnitTool   (this is the treeview user cntrol Iam mentioning above)
       Grid.Row="0"
       SelectedItemChanged="mBizUnitTool_SelectedItemChanged"
       x:Name="mBizUnitTool" />
                        <Button 
       Grid.Row="1"
       Width="100"
       Name="mBtnCloseBizUnitTool" 
       Click="mBtnCloseBizUnitTool_Click">Close</Button>
                    </Grid>
                </Popup>
            </Grid>
        </Button