Check for null item in BindingList<T>.InsertItem

Prevents `NullReferenceException` when inserting `null` object into `BindingList<T>` where `T` implements `INotifyPropertyChanged`
Test for bug 16902

Check insert of null item, in a BindingList<T> where T implements INotifyPropertyChanged does not cause NullReferenceException.
Merge pull request #844 from scottmcarthur/master

Check for null item in BindingList<T>.InsertItem