MvxFormsIosViewPresenter(AppDelegate, Window, FormsApp) throws InvalidCastException
## 🐛 Bug Report

<!--- Summary description of the bug --->
Create a new MvxFormsIosViewPresenter() would throw a InvalidCastException.

### Expected behavior
A MvxFormsIosViewPresenter is created without exception.

### Reproduction steps
Create a new MvxFormsIosViewPresenter() object. A sample project is available [here](https://github.com/mingxli/MvxFormsIosViewPresenterIssue).

### Configuration
Mac OS X 10.15.7
Visual Studio for Mac 8.8.8 (build 12)
Xamarin.Mac 7.4.0.4
Xamarin.iOS 14.10.0.4
MvvmCross 7.1.2
MvvmCross.Forms 7.1.2
Xamarin.Forms 4.8.0.1821

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