Listbox error for empty list before assigning items. Windows Phone

Listbox error for empty list before assigning items. Windows Phone

I have a list and want to assign the downloaded feeds her. Also wanted to
say that I am using the same code I've used in another app, but is giving
an error as few as this. The other works perfectly. I will post few as the
source of this. For if you can not stay too long.
private void carregaListas()
{
WebClient webClient = new WebClient();
webClient.DownloadStringCompleted += new
DownloadStringCompletedEventHandler(webClient_DownloadStringCompleted);
webClient.DownloadStringAsync(new
System.Uri("http://www.news-medical.net/syndication.axd?format=rss"));
}
private void webClient_DownloadStringCompleted(object sender,
DownloadStringCompletedEventArgs e)
{
if (e.Error != null)
{
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
stkLife.Visibility = Visibility.Visible;
stbOfLife.Begin();
});
}
else
{
// Save the feed into the State property in case the
application is tombstoned.
//gridProgressBar.Visibility = Visibility.Collapsed;
this.State["feed"] = e.Result;
UpdateFeedList(e.Result);
}
}
private void UpdateFeedList(string feedXML)
{
StringReader stringReader = new StringReader(feedXML);
XmlReader xmlReader = XmlReader.Create(stringReader);
SyndicationFeed feed = SyndicationFeed.Load(xmlReader);
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
feedListBox.ItemsSource = feed.Items;
});
}
Error: "Items collection must be empty before using ItemsSource."
XAML code:
<ListBox x:Name="feedListBox" Margin="0,0,-12,0"
SelectionChanged="feedListBox_SelectionChanged">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Margin="0,0,0,17">
<StackPanel.Background>
<SolidColorBrush Color="#FFC5C5C5"
Opacity="0.35"/>
</StackPanel.Background>
<TextBlock Text="{Binding Title.Text,
Converter={StaticResource
RssTextTrimmer}}" TextWrapping="Wrap"
Style="{StaticResource
PhoneTextExtraLargeStyle}"
Foreground="White" FontSize="30"/>
<TextBlock Text="{Binding Summary.Text,
Converter={StaticResource
RssTextTrimmer}}" TextWrapping="Wrap"
Style="{StaticResource
PhoneTextSubtleStyle}"
Foreground="#99FFFFFF"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
I Have used this sample:
http://code.msdn.microsoft.com/wpapps/RSS-Reader-Sample-1702775f