RecyclerView carga más con el error ProgressBar

Estoy tratando de crearRecyclerview onLoadMore conProgressBar en el fondo. He intentadoesta solución, pero método onLoadMore en miActivity nunca se llama. Por favor, ayúdame.

Aquí está miAdapter:

public class NewsAdapterTest extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    private ArrayList<NewsDataModel> dataSet;
    private Context context;

    private final int VIEW_ITEM = 1;
    private final int VIEW_PROG = 0;

    // The minimum amount of items to have below your current scroll position before loading more.
    private int visibleThreshold = 2;
    private int lastVisibleItem, totalItemCount;
    private boolean loading;
    private OnLoadMoreListener onLoadMoreListener;

    public NewsAdapterTest(ArrayList<NewsDataModel> data, Context context, RecyclerView recyclerView) {
        this.dataSet = data;
        this.context = context;


        if (recyclerView.getLayoutManager() instanceof LinearLayoutManager) {

            final LinearLayoutManager linearLayoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
            recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
                @Override
                public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                    super.onScrolled(recyclerView, dx, dy);

                    totalItemCount = linearLayoutManager.getItemCount();
                    lastVisibleItem = linearLayoutManager.findLastVisibleItemPosition();
                    if (!loading && totalItemCount <= (lastVisibleItem + visibleThreshold)) {
                        // End has been reached
                        // Do something
                        Log.i("newsAdaptertest", "onScrolled: End reached");
                        if (onLoadMoreListener != null) {
                            onLoadMoreListener.onLoadMore();
                        }
                        loading = true;
                    }
                }
            });
        }

    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        ExpandableTextView postMessage;
        TextView postTitle;
        TextView postSubTitle;
        ImageView authorImage;

        public MyViewHolder(View itemView) {
            super(itemView);
            this.authorImage = (ImageView) itemView.findViewById(R.id.header_image);
            this.postTitle = (TextView) itemView.findViewById(R.id.header_title);
            this.postSubTitle = (TextView) itemView.findViewById(R.id.header_subtitle);
            this.postMessage = (ExpandableTextView) itemView.findViewById(R.id.post_text_expand);
        }
    }

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

    @Override
    public int getItemViewType(int position) {
        return dataSet.get(position) != null ? VIEW_ITEM : VIEW_PROG;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        RecyclerView.ViewHolder vh;
        if (viewType == VIEW_ITEM) {
            View v = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.recycler_news, parent, false);

            vh = new MyViewHolder(v);
        } else {
            View v = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.recycler_progress, parent, false);

            vh = new ProgressViewHolder(v);
        }
        return vh;
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int listPosition) {
        if (holder instanceof MyViewHolder) {
            Binding Views...
        } else {
            ((ProgressViewHolder) holder).progressBar.setIndeterminate(true);
        }
    }

    public void setLoaded() {
        loading = false;
    }

    public void setOnLoadMoreListener(OnLoadMoreListener onLoadMoreListener) {
        this.onLoadMoreListener = onLoadMoreListener;
    }

    public interface OnLoadMoreListener {
        void onLoadMore();
    }

    public static class ProgressViewHolder extends RecyclerView.ViewHolder {
        public ProgressBar progressBar;

        public ProgressViewHolder(View v) {
            super(v);
            progressBar = (ProgressBar) v.findViewById(R.id.progressBar);
        }
    }

}

Y mi actividad:

private static NewsAdapterTest adapter;
    private LinearLayoutManager layoutManager;
    private static RecyclerView recyclerView;
    private ArrayList<NewsDataModel> data = new ArrayList<NewsDataModel>();

recyclerView = (RecyclerView) findViewById(R.id.recyclerview_news);
        layoutManager = new LinearLayoutManager(this);
        adapter = new NewsAdapterTest(data, NewsActivity.this, recyclerView);

        recyclerView.setHasFixedSize(false);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());

        adapter.setOnLoadMoreListener(new NewsAdapterTest.OnLoadMoreListener() {
            @Override
            public void onLoadMore() {
                Log.i(TAG, "onLoadMore called");
                //add progress item
                data.add(null);
                adapter.notifyItemInserted(data.size() - 1);

                Log.i(TAG, "Loading new data... (" + Integer.toString(loadNumber) + ") posts");
                loadPosts(startLoadPosition, loadNumber);
                startLoadPosition += loadNumber;

                //remove progress item
                data.remove(data.size() - 1);
                adapter.notifyItemRemoved(data.size());
                //add items one by one

                adapter.setLoaded();
                //or you can add all at once but do not forget to call mAdapter.notifyDataSetChanged();
            }
        });