Unhandled Exception related to ListView

I have a Xamarin.Forms.ListView bound to a simple ObservableCollection<string>. As my app outputs log messages ( strings), I add them to the ObservableCollection<string>. This works fine for an undetermined period of time -- the ListView adds rows as expected -- then I get the attached unhandled exception. I can't pin down what makes this happen. Any thoughts?

Running the latest stable Xamarin in Visual Studio, deploying to Motorola Droid Maxx.