#vulnerable code 
                                boolean add(Side side, long price, long quantity) {
        Long2LongRBTreeMap levels = getLevels(side);

        long size = levels.get(price);

        levels.put(price, size + quantity);

        return price == levels.firstLongKey();
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                OrderBook(long instrument) {
        this.instrument = instrument;

        this.bids = new Long2LongRBTreeMap(BidComparator.INSTANCE);
        this.asks = new Long2LongRBTreeMap(AskComparator.INSTANCE);
    }