Navigate to new page from code that doesn't derive from ContentPage

Hi everyone,

My question isn't as simple as the title suggests.. I'm aware that I can use the navigation stack to find out the top-most page and call the navigation event on that page.

This is what I was planning on doing..

            Page currentPage;
            int index = Application.Current.MainPage.Navigation.NavigationStack.Count - 1;
            currentPage = Application.Current.MainPage.Navigation.NavigationStack[index];

        //after getting the correct page..
            //await currentPage.Navigation.PushAsync(new SomePage());

My problem is that the index is returning as -1.
I believe there's some added complexity due to my page hierarchy/structure.

My Apps main page is a LoginPage - and once the user has successfully logged in, they are pushed to a Modal page, which then has the rest of the navigation taking place from there.
await Navigation.PushModalAsync(new NavigationPage(new MainMenu()));

**How can I find the current active page (that the user is viewing) - from a class that is not inheriting from a ContentPage, when the NavigationStack includes a modal page? **

I need to do this so that my static 'helper' class (which the platform specific iOS / Android code accesses) can be passed a string for the page name, and this helper class can resolve that name, and navigate to the page if it exists. The helper class is being accessed via platform-specific implementations after an event on the phone (push notification tapped on)

Any help would be greatly appreciated!
Cheers.