DockSite.ActiveWindow is null when combobox popup shows in child ToolWindow

DockSite.ActiveWindow become null after combobox click.

Build 12.1.0562 used.

Sample xaml to reproduce issue:

<Window x:Class="TestWpf.MainWindow"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking" Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>

<TextBlock Grid.Row="0" Text="{Binding ElementName=Site, Path=ActiveWindow}"/>

<docking:DockSite x:Name="Site"
Grid.Row="2"
ToolWindowsHaveTitleBars="True"
AreDocumentWindowsDestroyedOnClose="True"
Switcher="{x:Null}">

<docking:SplitContainer Orientation="Vertical">
<docking:SplitContainer Orientation="Horizontal">
<docking:ToolWindowContainer x:Name="_left" docking:DockSite.ControlSize="500, 500">
<docking:ToolWindow>
<TextBlock Text="DSDSDS"/>

</docking:ToolWindow>

</docking:ToolWindowContainer>

<docking:ToolWindowContainer x:Name="_right" docking:DockSite.ControlSize="130, 500">
<docking:ToolWindow Title="1">
<TextBlock Text="DSDSDS"/>

</docking:ToolWindow>

</docking:ToolWindowContainer>
</docking:SplitContainer>
<docking:ToolWindowContainer x:Name="_bottom" docking:DockSite.ControlSize="500, 100">
<docking:ToolWindow>
<ComboBox SelectedIndex="0" >
<ComboBox.Items>
<TextBlock Text="1"/>
<TextBlock Text="2"/>
</ComboBox.Items>
</ComboBox>

</docking:ToolWindow>
</docking:ToolWindowContainer>
</docking:SplitContainer>
</docking:DockSite>
</Grid>
</Window>