#vulnerable code 
                                @Test
    public void processRequest_UnRegisterProducer() throws Exception {
        brokerController.getProducerManager().registerProducer(group, clientChannelInfo);
        HashMap<Channel, ClientChannelInfo> channelMap = brokerController.getProducerManager().getGroupChannelTable().get(group);
        assertThat(channelMap).isNotNull();
        assertThat(channelMap.get(channel)).isEqualTo(clientChannelInfo);

        RemotingCommand request = createUnRegisterProducerCommand();
        RemotingCommand response = clientManageProcessor.processRequest(handlerContext, request);
        assertThat(response).isNotNull();
        assertThat(response.getCode()).isEqualTo(ResponseCode.SUCCESS);

        channelMap = brokerController.getProducerManager().getGroupChannelTable().get(group);
        assertThat(channelMap).isNull();
    } 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void processRequest_UnRegisterProducer() throws Exception {
        brokerController.getProducerManager().registerProducer(group, clientChannelInfo);
        Map<Channel, ClientChannelInfo> channelMap = brokerController.getProducerManager().getGroupChannelTable().get(group);
        assertThat(channelMap).isNotNull();
        assertThat(channelMap.get(channel)).isEqualTo(clientChannelInfo);

        RemotingCommand request = createUnRegisterProducerCommand();
        RemotingCommand response = clientManageProcessor.processRequest(handlerContext, request);
        assertThat(response).isNotNull();
        assertThat(response.getCode()).isEqualTo(ResponseCode.SUCCESS);

        channelMap = brokerController.getProducerManager().getGroupChannelTable().get(group);
        assertThat(channelMap).isNull();
    }