this.radRichTextBox.CommandExecuted += this.RadRichTextBox_CommandExecuted;

private void RadRichTextBox_CommandExecuted(object sender, CommandExecutedEventArgs e)
{
    if (e.Command is TabForwardCommand)
    {
        DocumentPosition caret = this.radRichTextBox.Document.CaretPosition;
        if (caret.IsPositionInsideTable)
        {
            DocumentSelection selection = this.radRichTextBox.Document.Selection;
            DocumentPosition start = selection.Ranges.First.StartPosition;
            DocumentPosition end = selection.Ranges.Last.EndPosition;
            Inline currentInline = start.GetCurrentInline();
            if (currentInline is PermissionRangeStart)
            {
                start.MoveToNext();
            }

            Inline previousInline = end.GetPreviousInline();
            if (previousInline is PermissionRangeEnd)
            {
                end.MoveToPrevious();
            }

            this.radRichTextBox.Document.Selection.SetSelectionStart(start);
            this.radRichTextBox.Document.Selection.AddSelectionEnd(end);
        }
    }
}

foreach (Span span in paragraphCopy.EnumerateChildrenOfType<Span>())
{
    if (string.IsNullOrEmpty(span.Text))
    {
        span.Text = " ";
    }
}