Application doesn't close after showing new separate window
## 🐛 Bug Report

I have an wpf application. Which contains one MainWindow and UserView which I open in a new separate window.
When I open only MainWindow and close it with X button the application terimnates it process. But when I open MainWindow and UserView, after closing UserView and MainWindow the application is still running, I can see process in task manager or in Visual Studio. It makes no difference whether I close that UserView with X button or with Navigation Service.

### Expected behavior
The application should terminates it process. 
### Reproduction steps
Open new separate window from MainViewModel.

`public class SignatureViewModel : `MvxNavigationViewModel`
`{`
`}`

`var user = await NavigationService.Navigate<UserViewModel, UserViewModelParameter, `UserWithGroups>(parameter);`

 `public class UserViewModel : MvxNavigationViewModel<UserViewModelParameter, UserWithGroups>`
`{`
}`

And closes it with X button or with Navigation service like below
 `await NavigationService.Close(this, user);`   

### Configuration

**Version:** 7.2

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


