UINavigationController Null in UIScrollView ScrollAnimationEnded event after ResignFirstResponder

AppDelegate.cs

using MonoTouch.Foundation;
using MonoTouch.UIKit;
using System.Drawing;

namespace PopTestApp
{
	[Register("AppDelegate")]
	public class AppDelegate : UIApplicationDelegate
	{
		UIWindow window;
		UINavigationController navigationController;

		public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
		{
			// Navigation controller
			navigationController = new UINavigationController(new FirstController());

			// Window
			window = new UIWindow(UIScreen.MainScreen.Bounds);
			window.RootViewController = navigationController;
			window.MakeKeyAndVisible();
			return true;
		}
	}

	public class FirstController : UIViewController
	{
		public FirstController() { 
			Title = "First"; 
			View.BackgroundColor = UIColor.Red; 
		}

		public override void TouchesBegan(NSSet touches, UIEvent evt)
		{
			// Push the next controller
			NavigationController.PushViewController(new SecondController(), false);
		}
	}

	public class SecondController : UIViewController
	{
		readonly UIScrollView ScrollView;
		readonly UITextField Field;
		readonly UIButton Button;

		public SecondController()
		{
			Title = "Second";
			View.BackgroundColor = UIColor.Blue;

			// Field will scroll and fail to pop the view when resigned
			Field = new UITextField(new RectangleF(15,50,260,40)) { BackgroundColor = UIColor.White };
			Field.ShouldReturn += (field) => {
				ScrollView.SetContentOffset(new PointF(0, 200), true);
				return true;
			};

			// Button will scroll and pop the view successfully
			Button = new UIButton(new RectangleF(15,100,260,40)) { };
			Button.SetTitle("Working Scroll & Pop", UIControlState.Normal);
			Button.TouchUpInside += (sender, e) => ScrollView.SetContentOffset(new PointF(0, 200), true);

			ScrollView = new UIScrollView { Frame = new RectangleF(new PointF(0, 0), View.Frame.Size), ContentSize = new SizeF(View.Frame.Width, View.Frame.Height + 1000) };
			ScrollView.ScrollAnimationEnded += (sender, e) => 
				// Pop to the root view controller
				// This action works after the scroll trigger by the button
				// This action fails (NavigationController is null) when resigning the UITextField - Why?
				NavigationController.PopToRootViewController(true);

			Add(ScrollView);
			ScrollView.Add(Field);
			ScrollView.Add(Button);
		}
	}
}