C# E_RUNTIME_SETVALUE exception

I'm working on a WP8.1 app and I'm getting an exception with message E_RUNTIME_SETVALUE.

The exception has no stack trace, and actual type is NullReferenceException:E_RUNTIME_SETVALUE

Its happening when I have an incremental loading list view fetching new items with the bottom of the list visible.

I'm also making use of OnContainerContentChanging method to incrementally load the data into ListView items - I assume it is somewhere in here that a value is being set incorrectly, although it only ever happens under there conditions.

What is the best way to debug this? Finding it very hard because no stack trace and the excpetion is thrown in the generated generated app.xaml.gc file.