Application Crashes and showing null exception when room database is empty and try to run this query

Query:

@Query("SELECT SUM(productPrice) FROM VitalCafeCart")
    LiveData<Integer> getSum();

and the result of this working fine when database have rows in it. i am using observer to get result.

here is the observer code.

LiveData<Integer> sum = AppDatabase.getInstance(mContext).atcDao().getSum();
            Observer<Integer> cartTotalObserver = new Observer<Integer>() {
                @Override
                public void onChanged(Integer integer) {

                        if (integer == 0) {
                            linearGotoCart.setVisibility(View.GONE);
                        } else {
                            shopDetailsTotalPrice.setText("Rs."+String.valueOf(integer));
                            linearGotoCart.setVisibility(View.VISIBLE);
                        }

                }
            };
            sum.observe(getActivity(), cartTotalObserver);