GSON unit test controller - failing

[ashok84munna]

I'm trying to write a unit test for a simple controller that takes GSON as input and writes to DB.
GSON because, the input has nested objects.

Controller implementation:

def create() {
    def artist = new Artist(request.GSON)
    if (!artist.save(flush: true)) {
        artist.errors.each {
            log.error("Error while creating Artist " + it)
        }
        render status: 500, layout: null
        return
    }
    response.status = 201
    render artist as GSON
}

Unit test:

@TestMixin(GsonUnitTestMixin)
@TestFor(ArtistController)
@Mock(Artist)
class ArtistControllerTests {
    void testCreate() {
        request.GSON = "{guid:123,"+
                                           "name: 'Akon',"+
                    "albums: ["+
                    "{guid:1,"+
                    "name:'album 1'"+
                    "}]}"
       controller.create()
       assert response.status == 201
      }
   }

Exception:
Cannot get property 'manyToOne' on null object at
def artist = new Artist(request.GSON) in the controller