Problem in implementing Admob in iOS project of Xamarin.Forms

I am trying to implement banner ads in xamarin forms but nothing shows up after executing this code. Kindly help me how do I show banner ads in xamarin.ios with Admob. Or what's wrong with this code?

AddViewRenderer.cs

[assembly: ExportRenderer(typeof(AdControlView), typeof(AdMobRenderer))]
namespace AdMobTesting.iOS
{
    public class AdMobRenderer : ViewRenderer<AdControlView,BannerView>
    {

        const string AdmobID = "ca-app-pub-7901894761393287/8231209126";

        BannerView adView;
        bool viewOnScreen;

        protected override void OnElementChanged(ElementChangedEventArgs<AdControlView> e)
        {
            base.OnElementChanged(e);

            if (e.NewElement == null)
                return;

            if (e.OldElement == null)
            {
                adView = new BannerView(AdSizeCons.SmartBannerPortrait)
                {
                    AdUnitID = AdmobID,
                    RootViewController = UIApplication.SharedApplication.Windows[0].RootViewController
                };

                adView.AdReceived += (sender, args) =>
                {
                    if (!viewOnScreen) this.AddSubview(adView);
                    viewOnScreen = true;
                };

                adView.LoadRequest(Request.GetDefaultRequest());
                base.SetNativeControl(adView);
            }
        }
    }
}

AppDelegate.cs

[Register("AppDelegate")]
    public partial class AppDelegate : global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate
    {
        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            global::Xamarin.Forms.Forms.Init();
            MobileAds.Configure("ca-app-pub-7901894761393287~7049155248");

            LoadApplication(new App());

            return base.FinishedLaunching(app, options);
        }
    }

AdMobTestingPage.Xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:AdMobTesting" x:Class="AdMobTesting.AdMobTestingPage"
xmlns:ui="clr-namespace:AdMobTesting;assembly:AdMobTesting"
    >
    <AbsoluteLayout>
        <ui:AdControlView AbsoluteLayout.LayoutBounds="0,0.9,1,0.1" AbsoluteLayout.LayoutFlags="All" />
    </AbsoluteLayout>
</ContentPage>

AdControlView.cs

 public class AdControlView : View
    {

    }