[Android] Xamarin.InAppBilling OnActivityResult not called

Hi everyone!

I tried to implement the Xamarin.InAppBilling Component for Android and I can get the available products from Google Play Store.
When I try to purchase one of these products and call serviceConnection.BillingHandler.BuyProduct(product) normally OnActivityResult should be called but nothing is happening. I bond all error events from the serviceConnection but nothing is fired there either...