#vulnerable code 
                                @Test
    public void testPublishReceiveWithQoS2() throws Exception {
        LOG.info("*** testPublishReceiveWithQoS2 ***");
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(false);
        m_client.connect(options);
        m_client.subscribe("/topic", 2);
        m_client.disconnect();

        //publish a QoS 2 message another client publish a message on the topic
        publishFromAnotherClient("/topic", "Hello MQTT".getBytes(), 2);
        m_callback.reinit();
        m_client.connect(options);
        
        assertNotNull(m_callback);
        assertNotNull(m_callback.getMessage());
        assertEquals("Hello MQTT", m_callback.getMessage().toString());
    } 
                            #location 17 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testPublishReceiveWithQoS2() throws Exception {
        LOG.info("*** testPublishReceiveWithQoS2 ***");
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(false);
        m_client.connect(options);
        m_client.subscribe("/topic", 2);
        m_client.disconnect();

        //publish a QoS 2 message another client publish a message on the topic
        publishFromAnotherClient("/topic", "Hello MQTT".getBytes(), 2);
        m_callback.reinit();
        m_client.connect(options);
        
        assertNotNull(m_callback);
        MqttMessage message = m_callback.getMessage(true);
        assertNotNull(message);
        assertEquals("Hello MQTT", message.toString());
    }