title: Mockito.when().thenReturn() doesn't work or returns null
body: <p>During the test there is a NullPointerException thrown. I tried to debug it and the only thing I worked out was that eventOptional is always null. Just as if Mockito.when().thenReturn() didn't work. Can anybody help? Here's my code for a tested service and for the test itself:</p>
  
  <pre class="lang-java prettyprint-override"><code>@Service
  public class EventService {
  
      @Autowired 
      public EventService(EventRepository eventRepository) {
          this.eventRepository = eventRepository;
      }
      //...
      public void updateEvent(EventDTO eventDTO) {
          Optional&lt;Event&gt; eventOptional = eventRepository.findOneById(eventDTO.getId());
  
          eventOptional.orElseThrow(() -&gt; new BadRequestException(EVENT_NOT_FOUND));
          //...
      }
  }
  </code></pre>
  
  <p>And the test class:</p>
  
  <pre class="lang-java prettyprint-override"><code>@RunWith(MockitoJUnitRunner.class)
  public class EventServiceTest {
  
      @Mock
      private EventRepository eventRepository;
      @InjectMocks
      private EventService eventService;
  
      private Event sampleEventFromDb;
  
      @Before
      public void setUp() throws Exception {
  
          MockitoAnnotations.initMocks(this);
      }
  
      @Test
      public void shouldUpdateEventTestAndWithProperTime() throws Exception {
          EventDTO eventDTOMock = Mockito.mock(EventDTO.class);
  
          sampleEventFromDb = Event.builder()
              .name("name")
              .startDateTime(LocalDateTime.now())
              .placeName("place")
              .description("description")
              .publicEvent(true)
              .owner(new User())
              .build();
  
          Mockito.when(eventRepository.findOneById(anyString())).thenReturn(Optional.of(sampleEventFromDb));
          Mockito.when(eventDTOMock.getId()).thenReturn("1");
  
          eventService.updateEvent(eventDTOMock); //NullPointerException
          //...
      }
  }
  </code></pre>