Issues with passing data between two ListViews in TabbedPages.

I have a two ListViews with one that should display a subset of the other's trough a second source list populated with context actions from the first ListView source list. I both tried creating a List<T> property in the target ListView class and in the model class to bind as the source but despite marking them as static I always get an error stating:

Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object

What should I do? Is this the best approach to my issue? Anyway, this is my code:

MODEL

public class Nodes
{
    #region Tracking List
    public class TrackingList
    {
        public static List<Item> Items { get; set; }
    }
    #endregion
}

LISTVIEW CODEBEHIND

#region Events
public void AddTracking(object sender, EventArgs e)
{
    //Retrieves the object bound to the cell raising the event
    var mi = (MenuItem)sender;
    var item = (Nodes.Item)mi.BindingContext;

    Nodes.TrackingList.Items.Add(item);
}
#endregion