how to add FixedPage to DocumentViewer

Hi, in .NET Core 3.1. it work fine:

<Window x:Class="WpfApp1.Window02"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:WpfApp02"  
        mc:Ignorable="d"  
        Title="Window02" Height="450" Width="800">  
  <Window.DataContext>  
    <local:ViewModel/>  
  </Window.DataContext>  
  <Grid>  
    <DocumentViewer Document="{Binding Doc}"/>  
  </Grid>  
</Window>  

-----------------------------------------------

using System.Windows;  
using System.Windows.Controls;  
using System.Windows.Documents;  
  
namespace WpfApp02  
{  
  public class ViewModel  
  {  
    public ViewModel()  
    {  
      Doc = RenderDocument();  
    }  
  
    public IDocumentPaginatorSource Doc { get; set; }  
  
    public FixedDocument RenderDocument()  
    {  
      FixedDocument fd = new FixedDocument();  
      PageContent pc = new PageContent();  
      FixedPage fp = new FixedPage();  
      TextBlock tb = new TextBlock();  
      // add some text to a TextBox object  
      tb.Text = "This is some test text";  
      // add the text box to the FixedPage  
      fp.Children.Add(tb);  
      // ,add the FixedPage to the PageContent   
      pc.Child = fp;  
      // add the PageContent to the FixedDocument  
      fd.Pages.Add(pc);  
      return fd;  
    }  
  }  
}