Edited Issue: Custom template on ChildWindow may throw an exception [17727]

Cherya asked for this issue at http://wpftoolkit.codeplex.com/discussions/350454.
 
To reproduce the issue, set the following Template in a ChildWindow:
 
<xctk:ChildWindow x:Name="Popup" WindowStartupLocation="Center" IsModal="False">
  <xctk:ChildWindow.Template>
    <ControlTemplate TargetType="{x:Type xctk:ChildWindow}">
      <Grid />
    </ControlTemplate>
  </xctk:ChildWindow.Template>
</xctk:ChildWindow>
 
When we set that template, an exception is thrown ('Object reference not set to an instance of an object.').