I am trying to allow a user to create a table inside of a RichTextBox. I can create a Grid inside of the RichTextBox, but I am having some issues with it.

I start with this XAML in the Grid.

<RichTextBox Name="TB1" AcceptsReturn="True">
    <Paragraph TextAlignment="Center">
        Hi everybody
    </Paragraph>
    <Paragraph>
        <InlineUIContainer>
            <Grid Background="Black">
                <Grid.RowDefinitions>
                    <RowDefinition Height="10" />
                    <RowDefinition Height="10" />
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="10" />
                    <ColumnDefinition Width="10" />
                </Grid.ColumnDefinitions>
            </Grid>
        </InlineUIContainer>
    </Paragraph>
    <Paragraph>
        How are you today?
    </Paragraph>
</RichTextBox>

Then when I get the XAML out using the Xaml property of the RichTextBox I get this XAML.

<Section xml:space="preserve" HasTrailingParagraphBreakOnPaste="False" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

<Paragraph FontSize="11" FontFamily="Portable User Interface" Foreground="#FF000000" FontWeight="Normal" FontStyle="Normal" FontStretch="Normal" TextAlignment="Center">
<Run Text="Hi everybody" />
</Paragraph>

<Paragraph FontSize="11" FontFamily="Portable User Interface" Foreground="#FF000000" FontWeight="Normal" FontStyle="Normal" FontStretch="Normal" TextAlignment="Left">
<Run />
</Paragraph>

<Paragraph FontSize="11" FontFamily="Portable User Interface" Foreground="#FF000000" FontWeight="Normal" FontStyle="Normal" FontStretch="Normal" TextAlignment="Left">
<Run Text="How are you today?" />
</Paragraph>
</Section>

Notice here that the Grid has turned into an empty Run element.

Anyone know why this happens?