#vulnerable code 
                                @Test
    public void pollerThreadTearsDownNodeAfterTestIsCompleted() throws IOException {

        // Supported desired capability for the test session
        Map<String, Object> requestedCapability = getCapabilitySupportedByDockerSelenium();

        // Start poller thread
        proxy.startPolling();

        // Mock the poller HttpClient to avoid exceptions due to failed connections
        proxy.getDockerSeleniumNodePollerThread().setClient(getMockedClient());

        // Get a test session
        TestSession newSession = proxy.getNewSession(requestedCapability);
        Assert.assertNotNull(newSession);

        // The node should be busy since there is a session in it
        Assert.assertTrue(proxy.isBusy());

        // We release the session, the node should be free
        newSession.getSlot().doFinishRelease();

        // After running one test, the node shouldn't be busy and also down
        Assert.assertFalse(proxy.isBusy());
        long sleepTime = proxy.getDockerSeleniumNodePollerThread().getSleepTimeBetweenChecks();
        await().atMost(sleepTime + 2000, MILLISECONDS).untilCall(to(proxy).isDown(), equalTo(true));
    } 
                            #location 21 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void pollerThreadTearsDownNodeAfterTestIsCompleted() throws IOException {

        // Supported desired capability for the test session
        Map<String, Object> requestedCapability = getCapabilitySupportedByDockerSelenium();

        // Start poller thread
        proxy.startPolling();

        // Mock the poller HttpClient to avoid exceptions due to failed connections
        proxy.getDockerSeleniumNodePollerThread().setClient(getMockedClient());

        // Get a test session
        TestSession newSession = proxy.getNewSession(requestedCapability);
        Assert.assertNotNull(newSession);

        // The node should be busy since there is a session in it
        Assert.assertTrue(proxy.isBusy());

        // We release the session, the node should be free
        WebDriverRequest request = mock(WebDriverRequest.class);
        HttpServletResponse response = mock(HttpServletResponse.class);
        when(request.getMethod()).thenReturn("DELETE");
        when(request.getRequestType()).thenReturn(RequestType.STOP_SESSION);

        newSession.getSlot().doFinishRelease();
        proxy.afterCommand(newSession, request, response);

        // After running one test, the node shouldn't be busy and also down
        Assert.assertFalse(proxy.isBusy());
        long sleepTime = proxy.getDockerSeleniumNodePollerThread().getSleepTimeBetweenChecks();
        await().atMost(sleepTime + 2000, MILLISECONDS).untilCall(to(proxy).isDown(), equalTo(true));
    }