Spring MongoRepository is Null

I have the following code which attempts to save a POJO object (Actor) into MongoDB using Spring Mongo Repository, but the repository object is always Null. I have followed multiple examples but mainly this one

The POJO class:

@Document(collection = "actors")
public class Actor 
{
  @Id
  private String id;
  ...
  //constructor
  //setters & getters
}

The repository:

public interface ActorRepository extends MongoRepository<Actor, String> 
{
  public Actor findByFNameAndLName(String fName, String lName);
  public Actor findByFName (String fName);
  public Actor findByLName(String lName);
}

The service that uses the repository:

@Service
public class ActorService 
{
  @Autowired
  private ActorRepository actorRepository;

  public Actor insert(Actor a)
  {
    a.setId(null);
    return actorRepository.save(a);
  }
} 

And I access the service from a REST controller class:

@RestController
public class Controllers 
{

  private static final Logger logger = Logger.getLogger(Controllers.class);
  private static final ApplicationContext ctx = new  AnnotationConfigApplicationContext(SpringMongoConfig.class);

  private ActorService actorService = new ActorService();

  @RequestMapping(value="/createActor", method=RequestMethod.POST)
  public @ResponseBody String createActor(@RequestParam(value = "fName") String fName,
        @RequestParam(value = "lName") String lName,
        @RequestParam(value = "role") String role)
  {
    return actorService.insert(new Actor(null,fName,lName,role)).toString();

  }

 ...
}

The error that I get is NullPointerException from this line: return actorRepository.save(a); in the ActorService.insert() method.

Any Idea why is this happening?

EDIT: Here is the Spring Configurations

@Configuration
public class SpringMongoConfig extends AbstractMongoConfiguration
{ 
    @Bean
    public GridFsTemplate gridFsTemplate() throws Exception 
    {
        return new GridFsTemplate(mongoDbFactory(), mappingMongoConverter());
    }

    @Override
    protected String getDatabaseName() 
    {
        return "SEaaS";
    }

    @Override
    @Bean
    public Mongo mongo() throws Exception 
    {
        return new MongoClient("localhost" , 27017 );
    }

    public @Bean MongoTemplate mongoTemplate() throws Exception 
    {
        return new MongoTemplate(mongo(), getDatabaseName());
    }    
}