output: #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));
      }
input: #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