Crash on iOS when utilizing Custom modal presentation
## 🐛 Bug Report

When utilizing the Modal Presentation Style of UIModalPresentationStyle.Custom, the viewcontroller's PresentationController is null, which is causing an application crash.

### Expected behavior
The Custom modal presentation doesn't cause an app crash.

### Reproduction steps
Use the Custom modal presentation style on an mvxviewcontroller and navigate to it.
`[MvxModalPresentation(ModalPresentationStyle = UIModalPresentationStyle.Custom)]`

### Configuration

**Version:** MvvmCross 8.x, no specific iOS version that I'm aware of.

**Platform:** 
- [X] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms
