Resource Resolver being returned as null via Sling Models Annotation

Question

Resource Resolver is being returned as null while injecting through sling models , let me know if anything i am missing :

I tried with :

@Model(adaptables = Resource.class)
public class Navigation {

   @Inject  @Source("sling-object")
   private ResourceResolver resourceResolver;

}

I also tried with:

@Model(adaptables = Resource.class)
public class Navigation {

  @Inject
  private ResourceResolver resourceResolver;

}

In both cases it was being returned as null and throwing a null pointer exception when ResourceResolver is being used .

Let me know what i may be missing to correct this error.