PopupButton: WindowsFormsHost in PopupContent doesn't work

Simple example of problem:

<Window x:Class="ActiProProblem.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"
xmlns:wf="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms" Title="MainWindow" Height="350" Width="525">
<DockPanel>
<shared:PopupButton DisplayMode="PopupOnly"
PopupAnimation="None"
IsEnabled="True"
Grid.Column="2"
Grid.Row="2"
Margin="1"
HorizontalAlignment="Left"
VerticalAlignment="Center"
HorizontalContentAlignment="Center">
<shared:PopupButton.PopupContent>
<DockPanel Background="Chartreuse"
Width="320"
Height="240">
<WindowsFormsHost Background="DeepPink" Width="320" Height="240">
<wf:TreeView Width="320"
Height="240"/>
</WindowsFormsHost>

</DockPanel>

</shared:PopupButton.PopupContent>
</shared:PopupButton>
</DockPanel>
</Window>

Run application, click popup button. Where is my WindowsFormsHost and its content?

Best regards - Marcin Szotka, InstalSoft