Locating the source of an UnhandledException

Is it possible that you have multiple threads making concurrent accesses to a collection that isn't thread-safe? Say, one thread removing an item while another is reading or adding? That would definitely cause this type of error. In general, the .NET collection classes (except for those in System.Collections.Concurrent) are not thread-safe. You can have multiple readers, but any modification of the collection is going to require exclusive access--protected by a lock or a reader/writer lock.

It's pretty safe to assume App.g.cs is not where the problem lies, so as you said, there isn't much to go on. So here's my hint to you: look at the inner exception. Those things are quite handy, and I'll be willing to bet you have an inner exception at least one layer deep.