The February update to the Silverlight Toolkit seems to have introduced a bug into the LongListSelector. The issue has been reported on CodePlex.
I run into this problem in my app because I have a page with a LongListSelector and in its OnNavigatedTo method I'm updating the selector's ItemsSource. This is necessary because the user can navigate to a settings page and toggle some settings which affect the data displayed in the LongListSelector. So, unless I update the ItemsSource each time, the changes are not displayed when the user navigates back to this page.
I'd like to dig into where exactly the NullReferenceException is being thrown within the Balance method, but cannot figure out how to tell Visual Studio (I'm using the express edition) where the toolkit source code is located. My project references the toolkit DLL from the GAC, but the actual source is located in another directory.
How can I tell Visual Studio where the source to step into is located?
Add the toolkit source project to your solution and then reference the project, not the assembly in the GAC.