Null pointer exception in Recommendations App, Google Play Services project

     import android.support.v7.widget.RecyclerView;
     import android.view.LayoutInflater;
     import android.view.View;
     import android.view.ViewGroup;
     import android.widget.ImageView;
     import android.widget.TextView;

     import com.joshbgold.recommendations.model.ActiveListings;
     import com.joshbgold.recommendations.model.Listing;
     import com.squareup.picasso.Picasso;

     import retrofit.Callback;
     import retrofit.RetrofitError;
     import retrofit.client.Response;

     public class ListingAdapter extends RecyclerView.Adapter<ListingAdapter.ListingHolder>
         implements Callback<ActiveListings>{

         private MainActivity activity;
         private LayoutInflater inflater;
         private ActiveListings activeListings;

         public ListingAdapter(MainActivity activity){
             this.activity = activity;
             inflater = LayoutInflater.from(activity);
    }

    @Override
    public ListingHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
        return new ListingHolder(inflater.inflate(R.layout.layout_listing, viewGroup, false));
    }

    @Override
    public void onBindViewHolder(ListingHolder listingHolder, int i) {
        final Listing listing = activeListings.results[i];
        listingHolder.titleView.setText(listing.title);
        listingHolder.titleView.setText(listing.price);
        listingHolder.titleView.setText(listing.Shop.shop_name); 

        Picasso.with((listingHolder.imageView.getContext()))
                .load(listing.Images[0].url_570xN)
                .into(listingHolder.imageView);
    }

    @Override
    public int getItemCount() {
        if(activeListings == null) {
            return 0;
        }
        if (activeListings.results == null){
            return 0;
        }
        return activeListings.results.length;
    }

    @Override
    public void success(ActiveListings activeListings, Response response) {
        this.activeListings = activeListings;
        notifyDataSetChanged();
        this.activity.showList();
    }

    @Override
    public void failure(RetrofitError error) {
        this.activity.showError();
    }

    public ActiveListings getActiveListings(){
        return activeListings;
    }

    public class ListingHolder extends RecyclerView.ViewHolder{

        public ImageView imageView;
        public TextView titleView;
        public TextView shopNameView;
        public TextView priceView;

        public ListingHolder(View itemView) {
            super(itemView);
            imageView = (ImageView) itemView.findViewById(R.id.listing_image);
            titleView = (TextView) itemView.findViewById(R.id.listing_title);
            shopNameView = (TextView) itemView.findViewById(R.id.listing_shop_name);
            priceView = (TextView) itemView.findViewById(R.id.listing_price);
        }
    }
}

import android.os.Parcel;
import android.os.Parcelable;

public class Shop implements Parcelable {

    public int shop_id;
    public String shop_name;

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(shop_id);
        dest.writeString(shop_name);
    }

    public static final Creator<Shop> CREATOR = new Creator<Shop>() {
        @Override
        public Shop createFromParcel(Parcel source) {
            Shop shop = new Shop();
            shop.shop_id = source.readInt();
            shop.shop_name = source.readString();
            return shop;
        }

        @Override
        public Shop[] newArray(int size) {
            return new Shop[size];
        }
    };
}

import android.os.Parcel;
import android.os.Parcelable;

public class Listing implements Parcelable {

    public int listing_id;
    public String title;
    public String description;
    public String price;
    public String url;
    public Shop Shop;
    public Image[] Images;

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(listing_id);
        dest.writeString(title);
        dest.writeString(description);
        dest.writeString(price);
        dest.writeString(url);
        dest.writeParcelable(Shop, 0);
        dest.writeParcelableArray(Images, 0);
    }

    public static final Creator<Listing> CREATOR = new Creator<Listing>() {
        @Override
        public Listing createFromParcel(Parcel source) {
            Listing listing = new Listing();
            listing.listing_id = source.readInt();
            listing.title = source.readString();
            listing.description = source.readString();
            listing.price = source.readString();
            listing.url = source.readString();
            listing.Shop = source.readParcelable(Shop.class.getClassLoader());
            listing.Images = (Image[]) source.readParcelableArray(Image.class.getClassLoader());
            return listing;
        }

        @Override
        public Listing[] newArray(int size) {
            return new Listing[size];
        }
    };
}