Delete registration ID when user deletes app for IOS. (Pushsharp)

Question

I am sending push notification for IOS with pushsharp. I want to delete users registration ID when they delete app from their device. In order to do this I am using FeedbackService but it is not working. I tried it with invalid registration ID and FeedbackService is null. How can I fix this? Here is my FeedBackService:

List<string> invalidRegistrationIds = new List<string>(); //List for invalid registration IDs
FeedbackService fbs = new FeedbackService(config);
fbs.FeedbackReceived += (string deviceToken, DateTime timestamp) => {
    invalidRegistrationIds.Add(deviceToken);
};
fbs.Check();