Problem with restoring layout

In my project I have only ToolWindows - I don't use DocumentWindows. All the windows are added programatically. In the xaml I declare only the main workspace (see below) and I want to have layout loaded from a separate file.

The problems is:

The windows which are docked into the main workspace ("mainContainer") don't invoke WindowOpened event. They are shown correctly, but no event is invoked, so I cannot i.e. setup my Windows menu (to show which are open, and which are not)

My xaml declaration:

<docking:DockSite x:Name="DockManager"
x:FieldModifier="public">
<docking:Workspace>
<docking:TabbedMdiHost x:Name="tabbedMdiHost">
<docking:TabbedMdiContainer x:Name="mainContainer">
</docking:TabbedMdiContainer>
</docking:TabbedMdiHost>
</docking:Workspace>
</docking:DockSite>