Check for null item in BindingList<T>.InsertItem

Fix for [bug 16902](https://bugzilla.xamarin.com/show_bug.cgi?id=16902).
Prevents `NullReferenceException` when a class implementing `INotifyPropertyChanged` is used as `T` in `System.ComponentModel.BindingList<T>`, and a `null` object is added to the collection.