Null Pointer Exception.. App crashes on clicking Hourly Button.

public class HourAdapter extends RecyclerView.Adapter<HourAdapter.HourViewHolder>{

    private Hour[] mHours;

    public HourAdapter(Hour[] hours){
        mHours = hours;
    }

    @Override
    public HourViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.hourly_list_item, parent, false);
        HourViewHolder viewHolder = new HourViewHolder(view);
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(HourViewHolder holder, int position) {

        holder.bindHour(mHours[position]);

    }

    @Override
    public int getItemCount() {
        return mHours.length;
    }

    public class HourViewHolder extends RecyclerView.ViewHolder {

        public TextView mTimeLabel,mSummarylabel,mTemperatureLabel;
        public ImageView mIconImageView;

        public HourViewHolder(View itemView) {
            super(itemView);

            mTimeLabel = (TextView) itemView.findViewById(R.id.timeHourLabel);
            mTemperatureLabel = (TextView) itemView.findViewById(R.id.temperatureHourLabel);
            mSummarylabel = (TextView) itemView.findViewById(R.id.summaryHourLabel);
            mIconImageView = (ImageView) itemView.findViewById(R.id.iconHourImageView);

        }

        public void bindHour(Hour hour){
               mTimeLabel.setText(hour.getHour());
               mTemperatureLabel.setText(hour.getTemperature() + ""); 
               mSummarylabel.setText(hour.getSummary());
               mIconImageView.setImageResource(hour.getIconId()); 
        }
    }
}