How to implement RtfText property for WinRT RichTextBoxAdv control?

/// <summary>
/// Represents the extension class for SfRichTextBoxAdv.
/// </summary>
public class SfRichTextBoxAdvExtension : SfRichTextBoxAdv
{
    #region Fields
    bool skipUpdating = false;
    #endregion
 
    #region Properties
    /// <summary>
    /// Gets or Sets the rtf text.
    /// </summary>
    public string RtfText
    {
        get
        {
            return (string)GetValue(RtfTextProperty);
        }
        set
        {
            SetValue(RtfTextProperty, value);
        }
    }
    #endregion
 
    #region Constructor
    /// <summary>
    /// Initializes the instance of SfRichTextBoxAdvExtension class.
    /// </summary>
    public SfRichTextBoxAdvExtension()
    {
        // Wires the ContentChanged event.
        this.ContentChanged += RTE_ContentChanged;
    }
    #endregion
 
    #region Static Dependency Properties
    /// <summary>
    /// Using as a backing store for RtfText dependency property to enable styling, animation etc.
    /// </summary>
    public static readonly DependencyProperty RtfTextProperty = DependencyProperty.Register("RtfText", typeof(string), typeof(SfRichTextBoxAdvExtension), new PropertyMetadata(string.Empty, new PropertyChangedCallback(OnRtfTextChanged)));
    #endregion
 
    #region Static Events
    /// <summary>
    /// Called when rtf text changed.
    /// </summary>
    /// <param name="obj"></param>
    /// <param name="e"></param>
    private static void OnRtfTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        SfRichTextBoxAdvExtension richTextBox = (SfRichTextBoxAdvExtension)obj;
        //Update the document with the Rtf.
        richTextBox.UpdateDocument((string)e.NewValue);
    }
    #endregion
 
    #region Events
    /// <summary>
    /// Called when content changes in SfRichTextBoxAdv.
    /// </summary>
    /// <param name="obj"></param>
    /// <param name="args"></param>
    void RTE_ContentChanged(object obj, ContentChangedEventArgs args)
    {
        if (this.Document != null)
        {
            // To skip internal updation of document on setting RtfText property.
            skipUpdating = true;
            Stream stream = new MemoryStream();
            // Saves the document as Rtf Stream.
            this.Save(stream, FormatType.Rtf);
            stream.Position = 0;
            // Reads the stream and assigned the string to RtfText property
            using (StreamReader reader = new StreamReader(stream))
            {
                this.RtfText = reader.ReadToEnd();
            }
            skipUpdating = false;
        }
    }
    #endregion
 
    #region Implementation
    /// <summary>
    /// Updates the document.
    /// </summary>
    /// <param name="rtfText">The Rtf text.</param>
    private void UpdateDocument(string rtfText)
    {
        // If Rtf text property is set internally means, skip updating the document.
        if (!skipUpdating && !string.IsNullOrEmpty(rtfText))
        {
            Stream stream = new MemoryStream();
            // Convert the Rtf string to byte array.
            byte[] bytes = Encoding.UTF8.GetBytes(rtfText);
            // Writes the byte array to stream.
            stream.Write(bytes, 0, bytes.Length);
            stream.Position = 0;
            //Load the Rtf stream.
            Load(stream, FormatType.Rtf);
        }
    }
    /// <summary>
    /// Disposes the instance.
    /// </summary>
    public void Dispose()
    {
        this.ContentChanged -= RTE_ContentChanged;
        ClearValue(RtfTextProperty);
        base.Dispose();
    }
    #endregion
}

<local:SfRichTextBoxAdvExtension x:Name="richTextBoxAdv" Grid.Row="2" RtfText="{Binding RtfText, Mode=TwoWay}" LayoutType="Continuous" EnableMiniToolBar="False" FontFamily="Arial" FontSize="12" DocumentTitle="Note 1"/>