xamarin.ios - Using Three20 TTPhotoViewController with MonoTouch

Question

I'm trying to use the Three20 TTPhotoViewController with MonoTouch. I've derived FacebookPhoto from TTPhoto and FacebookPhotoSource from TTPhotoSource and am now trying to invoke the TTPhotoViewController but I get the following exception when pushing the view controller:

Objective-C exception thrown. Name: NSInvalidArgumentException Reason: * -[NSPlaceholderString initWithFormat:locale:arguments:]: nil argument

I noticed that the monotouch bindings in this github project: https://github.com/mono/monotouch-bindings/tree/492f68c3c2007f0638452cc8a5a762556db224ba/Three20/binding were missing the photoAtIndex binding, so I added that and recompiled them, but I haven't been able to figure out why I am getting this exception.

Here is how I'm invoking the TTPhotoViewController:

List<Photo> photoList = FacebookGraphApi.Instance.GetAlbumPhotos(album.id);
List<FacebookPhoto> fbPhotoList = photoList.Select(x => new FacebookPhoto(x)).ToList();
var photos = new TTPhotoViewController();
photos.PhotoSource = new FacebookPhotoSource(fbPhotoList);
NavController.PushViewController(photos, true);

Here is the definition of the TTPhotoSource

    class FacebookPhotoSource : TTPhotoSource
    {
        List<FacebookPhoto> _photoList;

        public FacebookPhotoSource (List<FacebookPhoto> photoList)
        {
            _photoList = photoList;
            int i = 0;
            foreach (FacebookPhoto photo in photoList) {
                photo.PhotoSource = this;
                photo.Index = i++;
            }
        }

        public override string Title {
            get {
                return "Facebook Photos";
            }
            set  {
                throw new NotImplementedException();
            }
        }

        public override int NumberOfPhotos {
            get {
                return _photoList.Count;
            }
        }

        public override int MaxPhotoIndex {
            get {
                return _photoList.Count -1;
            }
        }

        public override TTPhoto PhotoAtIndex(int photoIndex)
        {
            return _photoList[photoIndex];
        }

    }

and here is the definition of the FacebookPhoto:

    class FacebookPhoto : TTPhoto
    {
        Photo _photo;
        public FacebookPhoto(Photo photo)
        {
            _photo = photo;
        }

        public override string Caption {
            get {
                if(_photo.name == null)
                    return "";
                return _photo.name;
            }
            set {
                throw new NotImplementedException();
            }
        }

        public override TTPhotoSource PhotoSource { get; set; }

        public override int Index { get; set; }

        public override SizeF Size {
            get {
                return new SizeF(_photo.width, _photo.height);
            }
            set {
                throw new NotImplementedException();
            }
        }

        public override string URLForVersion (int version)
        {
            switch (version) {
            case 4:
                return _photo.picture;
            default:
                return _photo.source;
            }
        }
    }