Can we programmatically create FlowDocumentReader from the content of RichTexBox?

Question: How can we create a FlowDocumentReader from the content of a RichTextBox?

I know we cannot assign the same logical child of a control to another control; nor can we assign null to the Document property of a RichTextBox. Hence the following code attempt fails. Are there workarounds or better solutions?

MainWindow.xaml:

<Window x:Class="WPFRichTextBxTest.MainWindow"
        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:WPFRichTextBxTest"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <DockPanel Name="mainPanel">
            <ToolBar Name="mainToolBar" Height="30" DockPanel.Dock="Top">
                <Button x:Name="btnTest" Content="Test" Click="btnTest_Click"/>
            </ToolBar>
            <RichTextBox Name="rtbTest" AcceptsTab="True"/>
            <FlowDocumentReader x:Name="fdReader"></FlowDocumentReader>
        </DockPanel>    
    </Grid>
</Window>

Code:

private void btnTest_Click(object sender, RoutedEventArgs e)
{
    FlowDocument flowDoc = rtbTest.Document;
    rtbTest.Visibility = Visibility.Collapsed;
    rtbTest.Document = null;
    fdReader.Document = flowDoc ;
}

You could set the Document property to a new empty FlowDocument:

private void btnTest_Click(object sender, RoutedEventArgs e)
{
    FlowDocument flowDoc = rtbTest.Document;
    rtbTest.Visibility = Visibility.Collapsed;
    rtbTest.Document = new FlowDocument(); //<--
    fdReader.Document = flowDoc;
}