Why am i getting an exception when running Windows 8.1

Question

I am developing an application in xamarin and android is running ok..but the following exception is raised whwn i run my Windows 8.1 project

An exception of type 'System.ArgumentNullException' occurred in Xamarin.Forms.Platform.WinRT.DLL but was not handled in user code

The exception comes from the following piece of code

public sealed partial class MainPage
{
    public MainPage()
    {
        this.InitializeComponent();

        LoadApplication(new MyApp.App());
    }
}

Stacktrace:

at Xamarin.Forms.Platform.WinRT.Platform.SetPage(Page newRoot) at Xamarin.Forms.Platform.WinRT.WindowsBasePage.LoadApplication‌​(Application application) at Marula.Windows.MainPage..ctor() at Marula.Windows.Marula_Windows_XamlTypeInfo.XamlTypeInfoProvi‌​der.Activate_4_MainP‌​age() at Marula.Windows.Marula_Windows_XamlTypeInfo.XamlUserType.Acti‌​vateInstance() InnerException: