cannot convert from 'MyNamespace.MyAppClass.MainActivity' to 'System.Func'

In my App I am using the nuget package Acr.Support. Everything worked fine until I had no MyAppClass. Since I surrounded the MainActivity with an Application Class I can´t use the command :

UserDialogs.Init(this);

Error message:
cannot convert from 'MyNamespace.MyAppClass.MainActivity' to 'System.Func<Android.App.Activity>'

Here is my code:

` namespace MyNamespace
{
[Application]
public class MyAppClass : Android.App.Application
{
public AmadeusAppClass(IntPtr handle, global::Android.Runtime.JniHandleOwnership transfer) : base(handle, transfer)
{
}

    public override void OnCreate()
    {
        //some stuff
    }

    [Activity(Label = "My App", MainLauncher = true, Theme = "@android:style/Theme.Holo.NoActionBar.Fullscreen")]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            UserDialogs.Init(this);
            //"this" gives me error message saying that I can´t convert from from 'MyNamespace.MyAppClass.MainActivity' to 'System.Func<Android.App.Activity>'
        }
    }
}

}`

Any idea what I am doing wrong?