Sending objects between layers

Question

I recently started my first C# course in school and we´re supposed to work with three layer architecture on our course project. The problem is that I don't seem to pass object between layers. I have a main GUI layer with a textbox input where the user shall pass in an URL to a podcast. This podcast is then supposed to move to the logic layer and then down to the data layer where a serialization process is started. I´m using a couple of classes to make this possible.

GUI layer: -"Main" This is where the user puts in a URL. Current code for add button:

    private void btnUrl_Click(object sender, RoutedEventArgs e)
    {
        FeedItem feed = new FeedItem();
        var url = tbUrl.Text;
        feed.Url = url;
        feedservice.Save(feed);
    }

Logic: -"FeedItem" this is what I´m trying to send.

Logic: -"FeedService" this is supposed to be the middle ground between GUI and data.

public class Feedservice

{
    internal static XmlSerializer Xml;

    private static List<FeedItem> Feedlista = new List<FeedItem>();

    private Dataklass<List<FeedItem>> feeds;

     public void Save(FeedItem nyFeed)
    {

        if (nyFeed.Url == null)
        {
            Console.Write("Du måste ge boxen en URL!");
        }
        Feedlista.Add(nyFeed);



        feeds.XMLFeed(Feedlista);
    }
}

Data: -"Dataklass" This is where I will start the serialization and path back to the listbox on "Main". When finished it´s supposed to be a podcast handler where the user can select a podcast and then play it with a media player of choice.

I can't seem to get the FeedItem object to be passed to the logic layer without getting a Nullexception error. Am I on the right path here or am I completely lost? If anyone got some kind of input I would appreciate it alot!