#vulnerable code 
                                @Override
    public void process(ComplexEventChunk complexEventChunk) {
        List<ComplexEventChunk<ComplexEvent>> outputEventChunks = new ArrayList<ComplexEventChunk<ComplexEvent>>();
        complexEventChunk.reset();
        synchronized (this) {
            complexEventChunk.reset();
            while (complexEventChunk.hasNext()) {
                ComplexEvent event = complexEventChunk.next();
                if (event.getType() == ComplexEvent.Type.TIMER) {
                    if (event.getTimestamp() >= scheduledTime) {
                        ComplexEventChunk<ComplexEvent> outputEventChunk = new ComplexEventChunk<ComplexEvent>(false);
                        for (ComplexEvent complexEvent : groupByKeyEvents.values()) {
                            outputEventChunk.add(cloneComplexEvent(complexEvent));
                        }
                        outputEventChunks.add(outputEventChunk);
                        scheduledTime += value;
                        scheduler.notifyAt(scheduledTime);
                    }
                } else if (event.getType() == ComplexEvent.Type.CURRENT) {
                    complexEventChunk.remove();
                    GroupedComplexEvent groupedComplexEvent = ((GroupedComplexEvent) event);
                    groupByKeyEvents.put(groupedComplexEvent.getGroupKey(), groupedComplexEvent.getComplexEvent());
                }
            }
        }
        for (ComplexEventChunk eventChunk : outputEventChunks) {
            sendToCallBacks(eventChunk);
        }
    } 
                            #location 13 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void process(ComplexEventChunk complexEventChunk) {
        List<ComplexEventChunk<ComplexEvent>> outputEventChunks = new ArrayList<ComplexEventChunk<ComplexEvent>>();
        complexEventChunk.reset();
        synchronized (this) {
            complexEventChunk.reset();
            while (complexEventChunk.hasNext()) {
                ComplexEvent event = complexEventChunk.next();
                if (event.getType() == ComplexEvent.Type.TIMER) {
                    tryFlushEvents(outputEventChunks, event);
                } else if (event.getType() == ComplexEvent.Type.CURRENT) {
                    complexEventChunk.remove();
                    tryFlushEvents(outputEventChunks, event);
                    GroupedComplexEvent groupedComplexEvent = ((GroupedComplexEvent) event);
                    groupByKeyEvents.put(groupedComplexEvent.getGroupKey(), groupedComplexEvent.getComplexEvent());
                }
            }
        }
        for (ComplexEventChunk eventChunk : outputEventChunks) {
            sendToCallBacks(eventChunk);
        }
    }