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

        long oldSize = levels.get(price);
        long newSize = oldSize + quantity;

        boolean onBestLevel = price == levels.firstLongKey();

        if (newSize > 0)
            levels.put(price, newSize);
        else
            levels.remove(price);

        return onBestLevel;
    } 
                            #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);
    }