Created Unassigned: WindowContainer [19756]

I have a Window with the following contents connected to a view model with a bool "IsOpen" property defaulted to false.

```
<Grid>
<TextBox Text="Background Control" Height="28" HorizontalAlignment="Left" Margin="112,106,0,0" VerticalAlignment="Top" />
<primitive:WindowContainer ModalBackgroundBrush="#70000000">
<control:ChildWindow IsModal="true" WindowStartupLocation="Center" WindowState="{Binding IsOpen, Converter={StaticResource BoolToState}}">
<TextBlock Text="Text" />
</control:ChildWindow>
</primitive:WindowContainer>
</Grid>
```
When the Window opens, the ModalBackgroudnColor is shown and the background TextBox is not accessible.

I would expect that since the only ChildWindow is closed that the WindowContainer ModalBackgroundColor would not be visible.

If I toggle "IsOpen" or start with the property defaulted to true, the update to false has the WindowContainer vanishing properly.