Xamarin.Forms: NullReferenceException using FreshMvvm's modal Pop methods

Question

I'm experiencing an error everytime I try to use FreshMvvm and the modal Pop methods (Android and UWP). Here is what I use:

Coming from a MasterDetail View I press "Open" to start a modal dialog (in this case with a new Navigation Stack):

private async Task StartNew()
{
    await CoreMethods.PushPageModelWithNewNavigation<NewPageModel>(new DataObject(true));
}

Then when I "Close" the dialog with

private async Task Close()
{
    await CoreMethods.PopModalNavigationService(false);
}

I immediatly encounter an unhandled exception telling me nothing but that I earned a

NullReferenceException.

This behaviour does also happen with other modal methods like

await CoreMethods.PushPageModel<NewPageModel>(true);
await CoreMethods.PopPageModel(true);

It does not happen if I use un-modal push/pop. After updating Xamarin.Forms from 3.0 to 3.3 it is at least gone for UWP. Android does not throw an unhandled exception but instead shows me this nice picture:

Does anybody have an idea?