using System;
using System.Collections.Generic;
using System.Linq;
using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace Hello_MultiScreen_iPhone
{
	[Register ("AppDelegate")]
	public partial class AppDelegate : UIApplicationDelegate
	{
		//---- declarations
		UIWindow window;
		UINavigationController rootNavigationController;
		
		// This method is invoked when the application has loaded its UI and it is ready to run
		public override bool FinishedLaunching (UIApplication app, NSDictionary options)
		{
			this.window = new UIWindow (UIScreen.MainScreen.Bounds);
			
			//---- instantiate a new navigation controller
			this.rootNavigationController = new UINavigationController();
			//---- instantiate a new home screen
			HomeScreen homeScreen = new HomeScreen();
			//---- add the home screen to the navigation controller (it'll be the top most screen)
			this.rootNavigationController.PushViewController(homeScreen, false);
			
			//---- set the root view controller on the window. the nav controller will handle the rest
			this.window.RootViewController = this.rootNavigationController;
			
			this.window.MakeKeyAndVisible ();
			
			return true;
		}
	}
}

using System;
using System.Collections.Generic;
using System.Linq;
using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace Hello_MultiScreen_iPhone
{
	[Register ("AppDelegate")]
	public partial class AppDelegate : UIApplicationDelegate
	{
		//---- declarations
		UIWindow window;
		
		// This method is invoked when the application has loaded its UI and it is ready to run
		public override bool FinishedLaunching (UIApplication app, NSDictionary options)
		{
			this.window = new UIWindow (UIScreen.MainScreen.Bounds);
			
			//---- instantiate a new navigation controller
			var rootNavigationController = new UINavigationController();
			//---- instantiate a new home screen
			HomeScreen homeScreen = new HomeScreen();
			//---- add the home screen to the navigation controller (it'll be the top most screen)
			rootNavigationController.PushViewController(homeScreen, false);
			
			//---- set the root view controller on the window. the nav controller will handle the rest
			this.window.RootViewController = rootNavigationController;
			
			this.window.MakeKeyAndVisible ();
			
			return true;
		}
	}
}

Remove class variable declaration for UINavigationController
Remove class variable declaration for UINavigationController
