Problems with UIApplicationDelegate versus FormsApplicationDelegate after update to unified IOS

After updating the Xamarin IOS project to unified IOS using Xamarin Studio 5.7.1 (build 17) xamarin.IOS 8.6.1.26 on a Mac, I get some compiling errors.

the system reports :

error CS0012: The type `MonoTouch.UIKit.UIImageView' is defined in an assembly that is not referenced. Consider adding a reference to assembly `monotouch, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065' 

and

error CS0012: The type `MonoTouch.UIKit.UIView' is defined in an assembly that is not referenced. Consider adding a reference to assembly `monotouch, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'

both errors occur on the same line:

        public class ImageCircleRenderer : ImageRenderer

This part is a copy of the article which is published a month or so ago about using circular images within your App.

the app delegate has another strange message:

AppDelegate.cs(21,75): error CS0012: The type `MonoTouch.UIKit.UIApplicationDelegate' is defined in an assembly that is not referenced. Consider adding a reference to assembly `monotouch, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'

While the class declaration within the AppDelegate is:

    [Register("AppDelegate")]
    public partial class AppDelegate : global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate

I'm using the following nuget packages:
Microsoft.Bcl
Microsoft.Bcl.Build
Microsoft.Net.Http
PCLStorage
Xam.Plugins.Settings
Xamarin.Forms
Xamarin.Insights

Reregistering the all but the Microsoft packages didn't solve the problem and as far as I can see, none of the csproj files have any mentioning of the Foundation class.

is there somebody who can help me with these problems?

Greetings,