#vulnerable code 
                                @Override
    public void process(StreamEvent event) {
        StreamEvent head = event;           //head of in events
        StreamEvent expiredEventTail;
        StreamEvent expiredEventHead;
        while (event != null) {
            processEvent(event);
            event = event.getNext();
        }
        //if window is expired
        if (count > length) {
            int diff = count - length;
            expiredEventTail = removeEventHead;
            for (int i = 1; i < diff; i++) {
                expiredEventTail = expiredEventTail.getNext();
            }
            expiredEventHead = removeEventHead;
            removeEventHead = expiredEventTail.getNext();
            expiredEventTail.setNext(null);
            head.addToLast(expiredEventHead);
            nextProcessor.process(head);                            //emit in events and remove events as expired events
            count = count - diff;
        } else {
            nextProcessor.process(head);                            //emit only in events as window is not expired
        }
    } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void process(StreamEvent event) {
        StreamEvent head = event;           //head of in events
        StreamEvent expiredEventTail;
        StreamEvent expiredEventHead;
        while (event != null) {
            processEvent(event);
            event = event.getNext();
        }
        //if window is expired
        if (count > length) {
            int diff = count - length;
            expiredEventTail = removeEventHead;
            for (int i = 1; i < diff; i++) {
                expiredEventTail = expiredEventTail.getNext();
            }
            expiredEventHead = removeEventHead;
            removeEventHead = expiredEventTail.getNext();
            expiredEventTail.setNext(null);
            addToLast(head, expiredEventHead);
            nextProcessor.process(head);                            //emit in events and remove events as expired events
            count = count - diff;
        } else {
            nextProcessor.process(head);                            //emit only in events as window is not expired
        }
    }