#fixed code
                                @Test
    public void unregisterProducer() throws Exception {
        producerManager.registerProducer(group, clientInfo);
        Map<Channel, ClientChannelInfo> channelMap = producerManager.getGroupChannelTable().get(group);
        assertThat(channelMap).isNotNull();
        assertThat(channelMap.get(channel)).isEqualTo(clientInfo);
        Channel channel1 = producerManager.findChannel("clientId");
        assertThat(channel1).isNotNull();
        assertThat(channel1).isEqualTo(channel);
        producerManager.unregisterProducer(group, clientInfo);
        channelMap = producerManager.getGroupChannelTable().get(group);
        channel1 = producerManager.findChannel("clientId");
        assertThat(channelMap).isNull();
        assertThat(channel1).isNull();

    }
#vulnerable code 
                                @Test
    public void unregisterProducer() throws Exception {
        producerManager.registerProducer(group, clientInfo);
        HashMap<Channel, ClientChannelInfo> channelMap = producerManager.getGroupChannelTable().get(group);
        assertThat(channelMap).isNotNull();
        assertThat(channelMap.get(channel)).isEqualTo(clientInfo);
        Channel channel1 = producerManager.findChannel("clientId");
        assertThat(channel1).isNotNull();
        assertThat(channel1).isEqualTo(channel);
        producerManager.unregisterProducer(group, clientInfo);
        channelMap = producerManager.getGroupChannelTable().get(group);
        channel1 = producerManager.findChannel("clientId");
        assertThat(channelMap).isNull();
        assertThat(channel1).isNull();

    } 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE