I have a question about the WindowedDocumentUIService

Hi.
I have a question about the WindowedDocumentUIService.
I am using the code below to pop up the pop-up window.
I always want to show a pop-up window in the center of my parents' window.
But it does not appear in the center of the parent window.
The pop-up window is always located in the center of the main window.
How can I implement the functionality I want?
-----------------------------------------Code----------------------------------------------

XAML
<MainWindow.xaml>  
  <dxmvvm:Interaction.Behaviors>  
          <dx:WindowedDocumentUIService x:Name="DocumentPopupService" WindowStartupLocation="CenterOwner" DocumentShowMode="Dialog" />  
          <dx:WindowedDocumentUIService x:Name="DocumentModalessPopupService" WindowStartupLocation="CenterOwner" DocumentShowMode="Default"/>  
  </dxmvvm:Interaction.Behaviors>  
<MainWindowViewModel>  

C#
void IMain.OpenDocumentPopup(...... object parent)  
{  
  IDocument doc = DocumentModalessPopupService.FindDocumentByIdOrCreate(id, sv =>  
  {  
      IDocument document = sv.CreateDocument(viewName, parameter, parent);   <<<<<<<<< parent  

      document.DestroyOnClose = true;  
      document.Title = title;  

      return document;  
  });  
  doc.Show();  
  DocumentModalessPopupService.ActiveDocument = doc;  
}  

<Sub1ViewModel>
  object parent = this;
  Main.OpenDocumentPopup(… parent);  <<<<<<<<< parent