Playground.WpfCore starts with the blank MainWindow
## 🐛 Bug Report

After the MvvmCross splash screen dismisses, the MainWindow is blank:
![image](https://user-images.githubusercontent.com/747563/127285124-9ea8e2eb-c0c8-4287-ba20-79cfe8550a61.png)

The Playground.Wpf run from the same sources is working as expected:

![image](https://user-images.githubusercontent.com/747563/127285490-93045e65-de97-424c-af06-3831fb82ff80.png)

### Expected behavior

The MainWindow should host the RootView content as the Playground.Wpf application.

### Reproduction steps

1. Clone the GitHub source codes.
2. Checkout the release/8.0.2 branch (but the same result is in the develop as well).
3. Set the Playground.WpfCore as the startup project.
4. Run in VS.

### Configuration

**Version:** 8.0.2

Run from the GitHub source codes of the release/8.0.2 branch without any mofications.

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