title: Spring WebFlux: block() method return null in Spring Data Reactive MongoDB
body: <p>i'm trying to learn project Reactor and have the problem.</p>
  
  <pre><code>@Test
  @DisplayName("check that standaloneUser fields are correct")
  void validateUserFields() {
      userService.save(standaloneUser).subscribe();
      assertEquals(userService.count().block(), Long.valueOf(1));
      User user = userService.findByEmail("test@gmail.com").block();
      assertNotNull(user);
      assertNotNull(user.getId());
      assertEquals(user.getFirstName(), "test");
      assertEquals(user.getLastName(), "test");
      assertNotEquals(user.getPassword(), "test");
      assertEquals(user.getRole(), Role.CANDIDATE);
      assertNotNull(user.getCreatedDate());
      assertNull(user.getStoppedDate());
      assertEquals(user.getEmail(), "test@gmail.com");
  }
  </code></pre>
  
  <p>Sometimes block() method returns null.
  Who can explain me this?
  Thanks</p>