Created Issue: nullReferenceException Contextmenu [10299]

I attached the contextmenu to my browser, and in scriptnotify the holded-link-uris are catched.

if ((App.Current as App).function == "" && e.Value.ToString() != null && (App.Current as App).IsValidUri(e.Value.ToString()) && (WebBrowser)sender == (App.Current as App).Browser)
{
usedUri = e.Value.ToString();

ContextMenu cm = new ContextMenu();
cm.Opened += new RoutedEventHandler(cm_Opened);
cm.Closed += new RoutedEventHandler(cm_Closed);



if (usedUri.EndsWith(".jpg") || usedUri.EndsWith(".png") || usedUri.EndsWith(".gif") || usedUri.EndsWith(".bmp"))
{
MenuItem menuItem0 = new MenuItem() { Header = "open in new tab", Tag = e.Value };
menuItem0.Click += new RoutedEventHandler(newTab_Click);
MenuItem menuItem1 = new MenuItem() { Header = "save image", Tag = e.Value };
menuItem1.Click += new RoutedEventHandler(saveImage_Click);
MenuItem menuItem2 = new MenuItem() { Header = "share", Tag = e.Value };
menuItem2.Click += new RoutedEventHandler(share_Click);

cm.Items.Add(menuItem0);
cm.Items.Add(menuItem1);
cm.Items.Add(menuItem2);
}
else
{
MenuItem menuItem0 = new MenuItem() { Header = "open in new tab", Tag = e.Value };
menuItem0.Click += new RoutedEventHandler(newTab_Click);
MenuItem menuItem2 = new MenuItem() { Header = "share", Tag = e.Value };
menuItem2.Click += new RoutedEventHandler(share_Click);

cm.Items.Add(menuItem0);
cm.Items.Add(menuItem2);
}

// ContextMenuService.SetContextMenu((App.Current as App).Browser, cm);
cm.IsZoomEnabled = true;
cm.VerticalOffset = mouseClickPosition.Y;
cm.IsOpen = true;
// AttachContextMenu();
}

now, i get a NullReferenceException every 2nd time i hold a link.
VS navigates to a new page, titled: "no source avaiable" and it's located here:
Microsoft.Phone.Controls.Toolkit.dll!Microsoft.Phone.Controls.ContextMenu.UpdateVisualStates(bool useTransitions) + 0x4f Bytes

so, what's the problem?
thanks for helping