#fixed code
                                @Test
    @StandardTierHubOnlyTest
    public void invokeMethodSucceed() throws Exception
    {
        testInstance.transportClient.open();

        for (int i = 0; i < testInstance.clientArrayList.size(); i++)
        {
            DeviceMethodStatusCallBack subscribedCallback = new DeviceMethodStatusCallBack();

            ((DeviceClient)testInstance.clientArrayList.get(i)).subscribeToDeviceMethod(new SampleDeviceMethodCallback(), null, subscribedCallback, null);

            long startTime = System.currentTimeMillis();
            while (!subscribedCallback.isSubscribed)
            {
                Thread.sleep(200);

                if (System.currentTimeMillis() - startTime > METHOD_SUBSCRIBE_TIMEOUT_MILLISECONDS)
                {
                    fail(buildExceptionMessage("Timed out waiting for device to subscribe to methods", testInstance.clientArrayList.get(i)));
                }
            }

            CountDownLatch countDownLatch = new CountDownLatch(1);
            RunnableInvoke runnableInvoke = new RunnableInvoke(methodServiceClient, testInstance.devicesList[i].getDeviceId(), METHOD_NAME, METHOD_PAYLOAD, countDownLatch);
            new Thread(runnableInvoke).start();
            countDownLatch.await(3, TimeUnit.MINUTES);

            MethodResult result = runnableInvoke.getResult();
            Assert.assertNotNull(buildExceptionMessage(runnableInvoke.getException() == null ? "Runnable returns null without exception information" : runnableInvoke.getException().getMessage(), testInstance.clientArrayList.get(i)), result);
            Assert.assertEquals(buildExceptionMessage("result was not success, but was: " + result.getStatus(), testInstance.clientArrayList.get(i)), (long)METHOD_SUCCESS,(long)result.getStatus());
            Assert.assertEquals(buildExceptionMessage("Received unexpected payload", testInstance.clientArrayList.get(i)), runnableInvoke.getExpectedPayload(), METHOD_NAME + ":" + result.getPayload().toString());
        }

        testInstance.transportClient.closeNow();
    }
#vulnerable code 
                                @Test
    @StandardTierHubOnlyTest
    public void invokeMethodSucceed() throws Exception
    {
        testInstance.transportClient.open();

        for (int i = 0; i < testInstance.clientArrayList.size(); i++)
        {
            ((DeviceClient)testInstance.clientArrayList.get(i)).subscribeToDeviceMethod(new SampleDeviceMethodCallback(), null, new DeviceMethodStatusCallBack(), null);

            CountDownLatch countDownLatch = new CountDownLatch(1);
            RunnableInvoke runnableInvoke = new RunnableInvoke(methodServiceClient, testInstance.devicesList[i].getDeviceId(), METHOD_NAME, METHOD_PAYLOAD, countDownLatch);
            new Thread(runnableInvoke).start();
            countDownLatch.await(3, TimeUnit.MINUTES);

            MethodResult result = runnableInvoke.getResult();
            Assert.assertNotNull(buildExceptionMessage(runnableInvoke.getException() == null ? "Runnable returns null without exception information" : runnableInvoke.getException().getMessage(), testInstance.clientArrayList.get(i)), result);
            Assert.assertEquals(buildExceptionMessage("result was not success, but was: " + result.getStatus(), testInstance.clientArrayList.get(i)), (long)METHOD_SUCCESS,(long)result.getStatus());
            Assert.assertEquals(buildExceptionMessage("Received unexpected payload", testInstance.clientArrayList.get(i)), runnableInvoke.getExpectedPayload(), METHOD_NAME + ":" + result.getPayload().toString());
        }

        testInstance.transportClient.closeNow();
    } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE