加载 RTF 文件后 WPF RichTextBox 不滚动

我正在尝试将富文本格式 (RTF) 文件加载到 WPF RichTextBox 中。当我执行加载时，看起来好像文件正在加载到 RichTextBox 中，但显示滚动条时没有可见的滑块框来滚动下载。滚动条不显示底部箭头，因此看起来滚动条的底部低于 RichTextBox 的显示区域。这防止用户能够向下滚动。我相信我要么缺少 RichTextBox 的 XAML 属性，要么我加载 RTF 文件的方式不对。请帮忙。提前致谢。

   <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
         </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
         </Grid.RowDefinitions>

        <StackPanel Grid.Row="0" Grid.RowSpan="5" 
                    Grid.Column="0" Grid.ColumnSpan="3" 
                    VerticalAlignment="Stretch" HorizontalAlignment="Stretch">

            <RichTextBox x:Name="LicenseRichTextBox" Margin="10"
                    VerticalAlignment="Stretch" HorizontalAlignment="Stretch"                 

                  IsReadOnly="True"  VerticalScrollBarVisibility="Visible">
            </RichTextBox>
        </StackPanel>
             <StackPanel Grid.Row="6"  Grid.Column="1" Grid.ColumnSpan="3" Margin="65,20,0,0" >
                <Button  x:Name="CloseButton" HorizontalAlignment="Left"
                      Width="90" Margin="-10,0,0,0"
                     Click="CloseButton_Click">
                    <StackPanel Orientation="Horizontal">
                        <Image Source="{StaticResource CloseButtonImageKey}" 
                           Margin="5,0,0,0" Height="20" Width="20" />
                        <TextBlock Padding="5,0,0,0" VerticalAlignment="Center"><Run Text="Close"/></TextBlock>
                    </StackPanel>
                </Button>
            </StackPanel>    
    </Grid>



   public  void LoadRTF()
    {
      const string EULA_Dir = @"Resources\EULA\EUlA.RTF";
      string currentDir = AppDomain.CurrentDomain.BaseDirectory;
      string PathToEULA = currentDir + EULA_Dir;

      if (File.Exists(PathToEULA))
      {
         LicenseRichTextBox.Selection.Load(new FileStream(PathToEULA, FileMode.Open), DataFormats.Rtf);
      }
      else
      {
        MessageBox.Show("Unable to locate the following file " + PathToEULA);
      }
    }