ListView BindingContext Error

Hi,
I'm creating a page with a ListView, and after selecting one of the objects and clicking on it, it should bring you to a details page for the item.
Everything works from the ListView, but I get an error after it tries to apply the BindingContext to the details page after an item is selected.
I'm not sure what I'm doing wrong.
Please advise...
I have packt all into a zip. Just put them into a portable project and adjust namespace, to get it to work...
The .Zip can be downloaded here: https://drive.google.com/open?id=0B3hQIMQb2QHmamFsZ0ZxVHc0LWM

(FYI: Error occurs in ListingPage line: 40 => page.BindingContext = ud)

public class ListingPage : ContentPage
{
private ObservableCollection users { get; set; }

    public ListingPage()
    {
        PreLoadData();
    }

    private async void PreLoadData()
    {
        try
        {
            App.ContextData.Sproc = "GetMeetingInfosOSP";
            string url = App.MainConnection + App.ContextData.Sproc + "/" + App.ContextData.UserID;
            Task<bool> task = LoadData(url);
            bool data = await task;

            if (data)
            {
                await PrepareData();

                var listView = new ListView();
                listView.RowHeight = 40;
                this.Title = "Kunden Info Liste";
                listView.ItemTemplate = new DataTemplate(typeof(DataCell));
                listView.ItemsSource = users;

                listView.ItemSelected += (sender, e) => {
                    UserData ud = (UserData)e.SelectedItem;

                    var page = new UserDataDetailsPage(ud);

//this line creates the error!!!! //this line creates the error!!!! //this line creates the error!!!! //this line creates the error!!!!
page.BindingContext = ud;

                    Navigation.PushAsync(page);
                };

                Content = listView;
            }
            else
            {
                await this.DisplayAlert("Fehler", "Die Daten konnten nicht geladen werden!", "OK");
            }
        }
        catch(Exception ex)
        {
            await this.DisplayAlert("Error", ex.Message, "OK");
        }
    }

    public async Task OnItemTapped(UserData user, object sender)
    {
        if (user == null) return;
        var page = new UserDataDetailsPage(user) { BindingContext = user };
        ((ListView)sender).SelectedItem = null; // de-select the row
        await Navigation.PushAsync(page);
    }

    private async Task<bool> LoadData(string url)
    {
        try
        {
            System.Json.JsonValue json = await JSONHandle.GetJSONDataAsync(url);
            App.ContextData.Data = json;
            return (json != null) ? true : false;
        }
        catch
        {
            return false;
        }
    }

    private async Task<bool> PrepareData()
    {
        try
        {
            users = new ObservableCollection<UserData>();
            JsonValue json = App.ContextData.Data;
            JsonArray jsonArray = (JsonArray)json[App.ContextData.Sproc + "Result"];

            for (int i = 0; i < jsonArray.Count; i++)
            {
                JsonObject jObj = (JsonObject)jsonArray[i];
                Newtonsoft.Json.Linq.JObject jo = Newtonsoft.Json.Linq.JObject.Parse(jObj.ToString());
                users.Add(DynamicDataUtility.CreateDataUser(jo));
            }

            return true;
        }
        catch (Exception ex)
        {
            await this.DisplayAlert("Error", "Error, Daten konnten nicht geladen werden!", "OK");
            return false;
        }
    }
}