Null pointer exception depending on statement chaining in Mockito?

I have a test method that is supposed to test the following method:

public class SelectionVariantUniqueConstraintValidatorImpl implements UniqueConstraintValidator {

    private SessionFactory sessionFactory;

    @Override
    public List<Criteria> buildValidationCriteria(Object entity, Serializable id, String[] propertyNames) {
        final SelectionVariant selectionVariant = (SelectionVariant) entity;
        final Session session = sessionFactory.getCurrentSession();
        final Criteria criteria = session.createCriteria(SelectionVariant.class);

        criteria.add(Restrictions.eq("client", selectionVariant.getClient()));
        criteria.add(Restrictions.eq("variant.variantName", selectionVariant.getVariant().getVariantName()));

        return new ArrayList<Criteria>(Arrays.asList(criteria));
    }

    @Required
    public void setSessionFactory(SessionFactory sessionFactory) {
        this.sessionFactory = sessionFactory;
    }
}

which runs fine. But if I modify the lines as follows:

final Criteria criteria = session.createCriteria(SelectionVariant.class).add(Restrictions.eq("client", selectionVariant.getClient()));
criteria.add(Restrictions.eq("variant.variantName", selectionVariant.getVariant().getVariantName()));

I will get:

java.lang.NullPointerException
    at com.innflow.ebtam.dao.hibernate.variant.SelectionVariantUniqueConstraintValidatorImpl.buildValidationCriteria(SelectionVariantUniqueConstraintValidatorImpl.java:35)

Why? What am I missing?

Here is my test class:

@RunWith(MockitoJUnitRunner.class)
public class SelectionVariantUniqueConstraintValidatorImplTest {

    SelectionVariantUniqueConstraintValidatorImpl selectionVariantUniqueConstraintValidator
            = new SelectionVariantUniqueConstraintValidatorImpl();

    @Mock
    private SessionFactory sessionFactory;
    @Mock
    private Session session;
    @Mock
    private Criteria criteria;

    @Before
    public void setUp() throws Exception {
        selectionVariantUniqueConstraintValidator.setSessionFactory(sessionFactory);
    }

    @Test
    public void testBuildValidationCriteria() throws Exception {
        Mockito.when(sessionFactory.getCurrentSession()).thenReturn(session);
        Mockito.when(session.createCriteria(SelectionVariant.class)).thenReturn(criteria);
        SelectionVariant selectionVariant = new SelectionVariant();
        Client client = new ClientBuilder().businessKey("HPD").description("desc").version(0).id(1L).build();
        selectionVariant.setClient(client);
        selectionVariantUniqueConstraintValidator.buildValidationCriteria(selectionVariant, null, null);
        Mockito.verify(criteria,Mockito.atLeastOnce()).add(Restrictions.eq("client", Mockito.any(Client.class)));
    }
}