Im ended this chapter correctly but i cant see my images!

Android Programming (3rd Edition) 26. Loopers, Handlers, and HandlerThread
1

hi when i run my app , I cant see the correct Activity . I think I have a problem with my recycle view but when I checking my code , there is no wrong code . I’m inflated my layouts and return my views .
I don’t know what’s wrong with it.
i have 3 layout :
{activity_fragment , id:fragment_container}
{fragment_photo_gallery , id : photo_recycler_view}
{list_item_gallery , id : item_image_view}


public class PhotoGalleryFragment extends Fragment {

    private static final String TAG = "PhotoGalleryFragment";

    private RecyclerView mPhotoRecyclerView;
    private List<GalleryItem> mItems = new ArrayList<>();
    private ThumbnailDownloader<PhotoHolder> mThumbnailDownloader;

    public static PhotoGalleryFragment newInstance(){
        return new PhotoGalleryFragment();
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
        new FetchItemsTask().execute();

        Handler responseHandler = new Handler();
        mThumbnailDownloader = new ThumbnailDownloader<>(responseHandler);
        mThumbnailDownloader.setThumbnailDownloadListener(
                new ThumbnailDownloader.ThumbnailDownloadListener<PhotoHolder>() {
                    @Override
                    public void onThumbnailDownloaded(PhotoHolder target, Bitmap thumbnail) {
                        Drawable drawable = new BitmapDrawable(getResources(),thumbnail);
                        target.bindDrawable(drawable);
                    }
                }
        );
        mThumbnailDownloader.start();
        mThumbnailDownloader.getLooper();
        Log.i(TAG,"background thread start");
    }


    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_photo_gallery,container,false);
        mPhotoRecyclerView=(RecyclerView) v.findViewById(R.id.photo_recycler_view);
        mPhotoRecyclerView.setLayoutManager(new GridLayoutManager(getActivity(),3));
        setupAdapter();
        return v;
    }

    public void setupAdapter() {
        if (isAdded()){
            mPhotoRecyclerView.setAdapter(new PhotoAdapter(mItems));
        }
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mThumbnailDownloader.quit();
        Log.i(TAG,"background thread destroy");
    }

    private static class PhotoHolder extends RecyclerView.ViewHolder{
        private ImageView mItemImageView;

        public PhotoHolder(View itemView) {
            super(itemView);
            mItemImageView=(ImageView) itemView.findViewById(R.id.item_image_view);
        }
        public void bindDrawable(Drawable drawable) {
            mItemImageView.setImageDrawable(drawable);
        }
    }



    private class PhotoAdapter extends RecyclerView.Adapter<PhotoHolder> {
        private List<GalleryItem> mGalleryItems;

        public PhotoAdapter (List<GalleryItem> galleryItems){
            mGalleryItems=galleryItems;
        }
        @Override
        public PhotoHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
            LayoutInflater inflater = LayoutInflater.from(getActivity());
            View view = inflater.inflate(R.layout.list_item_gallery,viewGroup,false);
            return new PhotoHolder(view);
        }
        @Override
        public void onBindViewHolder(PhotoHolder holder, int position) {
            GalleryItem galleryItem = mGalleryItems.get(position);
            Drawable placeHolder=getResources().getDrawable(R.drawable.bill_up_close);
            holder.bindDrawable(placeHolder);
            mThumbnailDownloader.queueThumbnail(holder,galleryItem.getUrl());
        }

        @Override
        public int getItemCount() {
            return mGalleryItems.size();
        }
    }

    private class FetchItemsTask extends AsyncTask<Void,Void,List<GalleryItem>> {
        @Override
        protected List<GalleryItem> doInBackground(Void... params) {
            return new FlickrFetchr().fetchItem();
        }

        @Override
        protected void onPostExecute(List<GalleryItem> Items) {
            mItems = Items;
            setupAdapter();
        }
    }
}