i am developing an app for windows phone 8 and i stuck on this code which visual studio showing invalid xaml. But Code compiles and works well. Invalid xaml Code is :

<DataTemplate x:Key="AddrBookItemTemplate">
    <StackPanel Margin="0,0,0,2" Orientation="Horizontal">
        <StackPanel Width="80" Orientation="Horizontal" Height="80">
            <Ellipse Margin="0" Height="70" Width="70" HorizontalAlignment="Left" Stroke="{x:Null}">
                <Ellipse.Fill>
                    <ImageBrush Stretch="Fill" ImageSource="{Binding imageBytes, Converter={StaticResource BytesToImageConverter}}"/>
                </Ellipse.Fill>
            </Ellipse>
        </StackPanel>
        <StackPanel Height="80" Margin="0" Width="380" HorizontalAlignment="Left">
            <TextBlock FontWeight="Bold"  Text="{Binding FirstName}" FontFamily="Segoe WP Semibold" FontSize="30" VerticalAlignment="Top" Margin="5,0,0,0" HorizontalAlignment="Left" />
            <TextBlock Text="{Binding Phone}" FontFamily="Segoe WP" FontSize="24" Foreground="{StaticResource PhoneTextBoxReadOnlyBrush}" Margin="5,0,0,-12" Width="320" HorizontalAlignment="Left" VerticalAlignment="Top"/>
        </StackPanel>
    </StackPanel>
</DataTemplate>

I am serializing image by converting it to byte, it works fine but if image is null it gives an error. code behind:

 if (e.Results != null)
 {
     List<AddressBook> source = new List<AddressBook>();

     foreach (var result in e.Results)
     {
         if (result.PhoneNumbers.FirstOrDefault() != null && result.GetPicture()!=null)
         {
             BitmapImage bmp = new BitmapImage(); 
             BitmapImage nullbmp = new BitmapImage();
             if (result.GetPicture() == null)
             {
                 bmp.UriSource = new Uri(@"/Images/ci2.png", UriKind.RelativeOrAbsolute);
             }
             else
             {
                 bmp.SetSource(result.GetPicture());
             }

             listobj.Add(new AddressBook()
             {
                 FirstName = result.DisplayName != null ? result.DisplayName : "",
                 imageBytes = AddressBook.imageConvert(bmp),
                 EmailAddress = "",
                 LastName = "",
                 Phone = result.PhoneNumbers.FirstOrDefault() != null ? result.PhoneNumbers.FirstOrDefault().PhoneNumber : "",
             });
         }
     }

Above code show an error "object reference not set to instance of an object". I want to show the default image (or color) in ellipse when image is null.What should I do?