How to focus the document window in WPF DockingManager by programmatically?

<syncfusion:DockingManager Grid.Row="1" x:Name="dockingManager" UseDocumentContainer="True">
  <ContentControl x:Name="Item1" syncfusion:DockingManager.Header="DockTab1" syncfusion:DockingManager.State="Document"/>
  <ContentControl x:Name="Item2" syncfusion:DockingManager.Header="DockTab2" syncfusion:DockingManager.State="Document"/>
  <ContentControl x:Name="Item3" syncfusion:DockingManager.Header="DockTab3" syncfusion:DockingManager.State="Document"/>
  <ContentControl x:Name="Item4" syncfusion:DockingManager.Header="DockTab4" syncfusion:DockingManager.State="Document"/>
  <ContentControl x:Name="Item5" syncfusion:DockingManager.Header="DockTab5" syncfusion:DockingManager.State="Document"/>
</syncfusion:DockingManager>

private void Button_Click_1(object sender, RoutedEventArgs e)
{
  for (var i = 0; i < dockingManager.Children.Count; i++)
  {
    var item = dockingManager.Children[i] as ContentControl;
    if (item.Name == "Item3")
    {
      dockingManager.ActivateWindow(item.Name);                    
      return;
    }
  }
}