Push view controller from iOS project when using NavigationPage.

I'm using a NavigationPage in my Xamarin.Forms project, and I can push pages fine in my PCL, but I need to push a view controller from within the iOS project and I can't seem to get a reference to the navigation controller. How is this done? AppDelegate.Window.RootViewController.NavigationController is null, I was expecting at runtime that this would be set.

AppDelegate FinishedLaunching:
Window.RootViewController = App.GetMainPage().CreateViewController();

Xamarin.Forms PCL App.cs:

public static Page GetMainPage ()
{
return new NavigationPage (new MemoriesPage ());
}