Exception when deserializing rafting document window

<Window x:Class="SerializeTest.MainWindow"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   xmlns:actiprodocking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
   Title="MainWindow" Height="350" Width="525">
   <Grid>
      <Grid.RowDefinitions>
         <RowDefinition Height="25"/>
         <RowDefinition Height="*"/>
      </Grid.RowDefinitions>

      <Button Content="Save" HorizontalAlignment="Left" VerticalAlignment="Top" Width="60" Click="SaveButton_Click"/>
      <Button Content="Load" HorizontalAlignment="Left" VerticalAlignment="Top" Width="60" Margin="70,0,0,0" Click="LoadButton_Click"/>

      <actiprodocking:DockSite Grid.Row="1" x:Name="MyDockSite" CanDocumentWindowsEditTitles="True" CanDocumentWindowsClose="False" CanDocumentWindowsRaft="True" UseHostedRaftingWindows="False">
         <actiprodocking:SplitContainer>
            <actiprodocking:Workspace>
               <actiprodocking:TabbedMdiHost IsCloseButtonOnTab="True" IsImageOnTab="True">
                  <actiprodocking:SplitContainer Orientation="Vertical">
                     <actiprodocking:TabbedMdiContainer>
                        <actiprodocking:DocumentWindow Name="doc1" Title="1"/>
                        <actiprodocking:DocumentWindow Name="doc2" Title="2"/>
                        <actiprodocking:DocumentWindow Name="doc3" Title="3"/>
                     </actiprodocking:TabbedMdiContainer>
                  </actiprodocking:SplitContainer>
               </actiprodocking:TabbedMdiHost>
            </actiprodocking:Workspace>
         </actiprodocking:SplitContainer>
      </actiprodocking:DockSite>
   </Grid>
</Window>

namespace SerializeTest
{
   public partial class MainWindow : Window
   {
      private DockSiteLayoutSerializer _layoutSerializer;
      private string _layoutXml;

      public MainWindow()
      {
         InitializeComponent();

         _layoutSerializer = new DockSiteLayoutSerializer();
         _layoutSerializer.SerializationBehavior = DockSiteSerializationBehavior.All;
         _layoutSerializer.DocumentWindowDeserializationBehavior =  DockingWindowDeserializationBehavior.AutoCreate;
      }

      private void SaveButton_Click(object sender, RoutedEventArgs e)
      {
         _layoutXml = _layoutSerializer.SaveToString(MyDockSite);
      }

      private void LoadButton_Click(object sender, RoutedEventArgs e)
      {
         _layoutSerializer.LoadFromString(_layoutXml, MyDockSite);
      }
   }
}