I want to test this method:

@Override
public GenreResponse fetchGenreById(Long genreId) {
    return convertGenreToGenreResponse(findGenreEntityByGenreId(genreId));
}

private GenreResponse convertGenreToGenreResponse(Genre genre) {
    return modelMapper.map(genre, GenreResponse.class);
}

private Genre findGenreEntityByGenreId(long genreId) {
    return genreRepository.findByGenreId(genreId)
        .orElseThrow(() -> new ObjectNotFoundException("Genre with " + genreId + " id not found!"));
}

I wrote a test for this method, but it doesn’t work, and I can’t figure out why.
This is my test:

@ExtendWith(MockitoExtension.class)
class GenreCrudServiceTest {

@Mock
private ModelMapper mockModelMapper;

@Mock
private GenreRepository mockGenreRepository;

private GenreCrudService genreCrudService;

@BeforeEach
void setUp() {
    MockitoAnnotations.openMocks(this);

    genreCrudService = new GenreCrudServiceImpl(mockModelMapper, mockGenreRepository);
}

@Test
public void GenreCrudService_FetchGenreById_ReturnsGenreResponse() {
    // Arrange
    long genreId = 1;

    Genre genre = Genre.builder()
        .genreId(genreId)
        .title("testTitle")
        .build();

    Mockito.when(mockGenreRepository.findByGenreId(any(Long.class))).thenReturn(Optional.of(genre));

    // Act
    GenreResponse genreResponse = genreCrudService.fetchGenreById(genreId);

    // Assert
    Assertions.assertThat(genreResponse).isNotNull();
}

But my test doesn’t work, it gives me an error:

Expecting actual not to be null
java.lang.AssertionError: 
Expecting actual not to be null

The error occurs on this line:

Assertions.assertThat(genreResponse).isNotNull();

I can’t figure out why I’m getting null returned, because here I’ve defined:

Mockito.when(mockGenreRepository.findByGenreId(any(Long.class))).thenReturn(Optional.of(genre));

that when searching for a genre by id, a specific Genre will be returned, and further method should convert it to GenreResponse