#vulnerable code 
                                @Test
    public void parsePayloadReturnsBytesForSpecifiedTopic(@Mocked final Mqtt mockMqtt) throws IOException
    {
        //arrange
        MqttDeviceTwin testTwin = new MqttDeviceTwin();

        String insertTopic = "$iothub/twin/"+ anyString;
        final byte[] insertMessage = {0x61, 0x62, 0x63};
        ConcurrentSkipListMap<String, byte[]> testMap = new ConcurrentSkipListMap<String, byte[]>();
        testMap.put(insertTopic, insertMessage);
        Deencapsulation.setField(mockMqtt, "allReceivedMessages", testMap);

        //act
        byte[] parsedPayload = testTwin.parsePayload(insertTopic);

        //assert
        assertNotNull(parsedPayload);
        assertEquals(insertMessage.length, parsedPayload.length);
        for (int i = 0 ; i < parsedPayload.length; i++)
        {
            assertEquals(parsedPayload[i], insertMessage[i]);
        }

    } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void parsePayloadReturnsBytesForSpecifiedTopic(@Mocked final Mqtt mockMqtt) throws IOException
    {
        //arrange
        MqttDeviceTwin testTwin = new MqttDeviceTwin();

        String insertTopic = "$iothub/twin/"+ anyString;
        final byte[] insertMessage = {0x61, 0x62, 0x63};
        ConcurrentSkipListMap<String, byte[]> testMap = new ConcurrentSkipListMap<String, byte[]>();
        testMap.put(insertTopic, insertMessage);
        Deencapsulation.setField(mockMqtt, "allReceivedMessages", testMap);

        //act
        byte[] parsedPayload = Deencapsulation.invoke(testTwin, "parsePayload", insertTopic);

        //assert
        assertNotNull(parsedPayload);
        assertEquals(insertMessage.length, parsedPayload.length);
        for (int i = 0 ; i < parsedPayload.length; i++)
        {
            assertEquals(parsedPayload[i], insertMessage[i]);
        }

    }