How to access the TabOrderIndexInDockMode of the DockWindow in DockingManager?

<Window x:Class="WpfDockingDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:syncfusion ="http://schemas.syncfusion.com/wpf"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
  <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <syncfusion:DockingManager Name="Docking" UseNativeFloatWindow="True">
        <ContentControl syncfusion:DockingManager.Header="Dock1" Name="Dock1" ></ContentControl>
        <ContentControl syncfusion:DockingManager.Header="Dock2" syncfusion:DockingManager.TargetNameInDockedMode="Dock3" syncfusion:DockingManager.SideInDockedMode="Tabbed" Name="Dock2" ></ContentControl>
        <ContentControl syncfusion:DockingManager.Header="Dock3" Name="Dock3" ></ContentControl>
        <ContentControl syncfusion:DockingManager.Header="Dock4" syncfusion:DockingManager.TargetNameInDockedMode="Dock3" syncfusion:DockingManager.SideInDockedMode="Tabbed" Name="Dock4" ></ContentControl>
        <ContentControl syncfusion:DockingManager.Header="Dock5" Name="Dock5" syncfusion:DockingManager.TargetNameInDockedMode="Dock3" syncfusion:DockingManager.SideInDockedMode="Tabbed"></ContentControl>
    </syncfusion:DockingManager>
    <Button Content="GetIndex of ActiveWindow" Grid.Row="1" VerticalAlignment="Bottom" Click="Button_Click"/>
    </Grid>
    </Grid>
</Window>

namespace DockingDemo
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml.
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Docking.Loaded += new RoutedEventHandler(Docking_Loaded);
        }
        void Docking_Loaded(object sender, RoutedEventArgs e)
        {
            Docking.ActivateWindow("Dock1");
        }
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            int d = 0;
            DockedElementTabbedHost host = DockingManager.GetHost(Docking.ActiveWindow, DockingManager.GetState(Dock3));
            if (host != null && host.InternalTabControl != null)
                d = host.InternalTabControl.Items.IndexOf(Docking.ActiveWindow);         MessageBox.Show(DockingManager.GetState(Docking.ActiveWindow).ToString() + " Index of " + DockingManager.GetHeader(Docking.ActiveWindow).ToString() + " is " + d.ToString());
        }
    }   
}