#vulnerable code 
                                @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        SocketChannel channel = (SocketChannel) ctx.channel();
        Attribute<Node> nodeAttribute = channel.attr(key);
        Node node = nodeAttribute.get();
        String nodeId = node == null ? null : node.getId();
        Log.debug("---------------------- client channelRegistered -----------" + nodeId);

        Map<String, Node> nodes = null;
        //Map<String, Node> nodes = getNetworkService().getNodes();
        // If a node with the same IP already in nodes, as a out node, can not add anymore
        for (Node n : nodes.values()) {
            //both ip and port equals , it means the node is myself
            if (n.getIp().equals(node.getIp()) && !n.getPort().equals(node.getSeverPort())) {
                Log.debug("----------------------client: it already had a connection: " + n.getId() + " type:" + n.getType() + ", this connection: " + nodeId + "---------------------- ");
                ctx.channel().close();
                return;
            }
        }
    } 
                            #location 12 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        SocketChannel channel = (SocketChannel) ctx.channel();
        Attribute<Node> nodeAttribute = channel.attr(key);
        Node node = nodeAttribute.get();
        String nodeId = node == null ? null : node.getId();
        Log.debug("---------------------- client channelRegistered -----------" + nodeId);

        Map<String, Node> nodes = nodeManager.getNodes();
        //Map<String, Node> nodes = getNetworkService().getNodes();
        // If a node with the same IP already in nodes, as a out node, can not add anymore
        for (Node n : nodes.values()) {
            //both ip and port equals , it means the node is myself
            if (n.getIp().equals(node.getIp()) && !n.getPort().equals(node.getSeverPort())) {
                Log.debug("----------------------client: it already had a connection: " + n.getId() + " type:" + n.getType() + ", this connection: " + nodeId + "---------------------- ");
                ctx.channel().close();
                return;
            }
        }
    }