Q:

User control with richTextBox, bindable richTextBox

I try make user control with richTextBox because I need bindable richTextbox. 
I found some solution here: Richtextbox wpf binding. 
I would like to use solution of Arcturus.  Create user control with richTextBox control and use dependency property. 
In XAML I have only richTextBox control:
<UserControl x:Class="WpfApplication2.BindableRichTextBoxControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <RichTextBox Name="RichTextBox" Grid. Row="0"/>    
    </Grid>
</UserControl>

In CodeBehind:
public partial class BindableRichTextBoxControl : UserControl
{
    public static readonly DependencyProperty DocumentProperty =
    DependencyProperty. Register("Document", typeof(FlowDocument), typeof(BindableRichTextBoxControl), 
    new PropertyMetadata(OnDocumentChanged));

    public FlowDocument Document
    {
        get { return (FlowDocument)GetValue(DocumentProperty); }
        set { SetValue(DocumentProperty, value); }
    }

    private static void OnDocumentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var control = (BindableRichTextBoxControl)d;
        if (e.NewValue == null)
            control. RichTextBox. Document=new FlowDocument();

        //? 
        control. RichTextBox. Document = document;
    }

    public BindableRichTextBoxControl()
    {
        InitializeComponent();
    }
}

I am little confuse with last line in OnDocumentChanged method.  
        control. RichTextBox. Document = document;

I can’t identify what is varibale document. 

A:

I think he means this:
private static void OnDocumentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    RichTextBoxControl control = (RichTextBoxControl) d;
    if (e.NewValue == null)
        control. RTB.Document = new FlowDocument(); //Document is not amused by null :)
    else
        control. RTB.Document = e.NewValue;
}

but I recommend you leave a comment on his original answer.