#fixed code
                                @Override
    public NetworkEventResult process(BaseEvent event, Node node) {
        GetNodeEvent getNodeEvent = (GetNodeEvent) event;

//        String key = event.getHeader().getEventType() + "-" + node.getIp();
//        if (cacheService.existEvent(key)) {
//            getNetworkService().removeNode(node.getId());
//            return null;
//        }
//        cacheService.putEvent(key, event, false);

        List<Node> list = getAvailableNodes(getNodeEvent.getLength(), node.getId());
        NodeEvent replyEvent = new NodeEvent(list);
        return new NetworkEventResult(true, replyEvent);
    }
#vulnerable code 
                                @Override
    public NetworkEventResult process(BaseEvent event, Node node) {
        GetNodeEvent getNodeEvent = (GetNodeEvent) event;

        String key = event.getHeader().getEventType() + "-" + node.getIp();
        if (cacheService.existEvent(key)) {
            getNetworkService().removeNode(node.getId());
            return null;
        }
        cacheService.putEvent(key, event, false);

        List<Node> list = getAvailableNodes(getNodeEvent.getLength(), node.getId());
        NodeEvent replyEvent = new NodeEvent(list);
        return new NetworkEventResult(true, replyEvent);
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE