Null Reference Exception when Navigating
## 🐛 Bug Report

I was getting a NRE when navigating to a page I had just added after a bit of debug I noticed I had a base type of MvxContentView rather than MvxContentPage

### Expected behavior

I know it was my mistake but maybe catch the null and provide the developer with a informative exception.

`MvxFormsPagePresenter.cs`

```
        public virtual Page CreatePage(Type viewType, MvxViewModelRequest request, MvxBasePresentationAttribute attribute)
        {
            var page = Activator.CreateInstance(viewType) as Page;
```
page is null and should be checked.

### Reproduction steps

Just navigate to a page which has a base type of MvxContentView

### Configuration

**Version:** 8.0.2

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