Null Activity after using Xamarin FilePicker in ViewModel
## 🐛 Bug Report

I'm using Xamarin FilePicker in a ViewModel to get a file. After that I'm using the IAppNavigationService to navigate to another ViewModel. The navigation service crashes because the Activity is null. This issue affects Android only, the same ViewModel is working correctly for iOS.

Code (trimmed down):
```C#
public IMvxAsyncCommand AttachDocumentCommand => new MvxAsyncCommand(
            async () =>
            {
                var result = await FilePicker.PickAsync(PickOptions.Default);

                var fileDetails = new ViewAttachementParameterModel() {
                    FResult = result
                };
                
                await NavigationService.Navigate<ViewAttachmentViewModel, ViewAttachementParameterModel>(fileDetails);
            });
```

The Navigate method is throwing an exception [Exception.txt](https://github.com/MvvmCross/MvvmCross/files/6750601/Exception.txt).

It could be related to #3455 

### Expected behavior

The navigation method should navigate normally to the destination ViewModel.

### Reproduction steps

1. Create an Android View that inherits from MvxDialogFragment and create its ViewModel
2. Add a command in the ViewModel to get a file from Xamarin FilePicker and then navigate to another view model
3. Add the required code to navigate to this Android View and execute the command

**Version:** 7.1.2

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