问AvalonDock DocumentPane未正确恢复

EN

<AvalonDock:DockingManager Grid.Row="1"
                               x:Name="MainDockingManager"
                               Loaded="MainDockingManager_Loaded">
        <AvalonDock:ResizingPanel Orientation="Vertical">
            <AvalonDock:ResizingPanel Orientation="Horizontal">
                <AvalonDock:DockablePane x:Name="LeftDockablePane"
                                         AvalonDock:ResizingPanel.ResizeWidth="250" />
                <AvalonDock:ResizingPanel Orientation="Vertical">
                    <AvalonDock:DocumentPaneResizingPanel>
                        <AvalonDock:DocumentPane x:Name="documentPane"/>
                    </AvalonDock:DocumentPaneResizingPanel>
                    <AvalonDock:DockablePane x:Name="BottomDockablePane"
                                             AvalonDock:ResizingPanel.ResizeHeight="200" />
                </AvalonDock:ResizingPanel>
            </AvalonDock:ResizingPanel>
        </AvalonDock:ResizingPanel>
    </AvalonDock:DockingManager>

    void MainDockingManager_Loaded( object sender, RoutedEventArgs e )
    {
        _layoutStateFilePath = Path.Combine( ApplicationSettingsService.UserSettingsDirectory, LayoutStateFileName );

        if ( File.Exists( _layoutStateFilePath ) )
        {
            MainDockingManager.RestoreLayout( _layoutStateFilePath );
        }
    }

    private void ApplicationViewsWindow_Closing( object sender, CancelEventArgs e )
    {
        MainDockingManager.SaveLayout( _layoutStateFilePath );
    }

documentPane.IsMainDocumentPane
true

documentPane.IsMainDocumentPane
null
MainDockingManager.MainDocumentPane.GetHashCode()
16306004
documentPane.GetHashCode()
15125992

MainDockingManager.MainDocumentPane = documentPane;