Android respon null saat request api dengan retrofit

Misi bang mau tanya, sudah coba browsing dll. dan mengulang tutorial android tetap sama responnya null saat di tampilkan ke TextView.

ini kode di OverviewFragment.kt

class OverviewFragment : Fragment() {
    private lateinit var recycleView: RecyclerView
    private lateinit var viewAdapter: RecyclerView.Adapter<*>
    private lateinit var viewManager: RecyclerView.LayoutManager
    private lateinit var viewModel: MyViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        val binding : FragmentOverviewBinding = DataBindingUtil.inflate(inflater, R.layout.fragment_overview, container, false)
        viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
        binding.setLifecycleOwner(this)

        binding.text1.text = viewModel.res.value.toString()

        return binding.root
    }
}

ini kodenya di viewModel

class MyViewModel : ViewModel() {
    private var _res = MutableLiveData<String>()
    val res: LiveData<String>
        get() = _res

    init {
        GithubAPI.retrofitService.showList().enqueue(object: Callback<String>{
            override fun onResponse(call: Call<String>?, response: Response<String>?) {
                _res.value = response!!.body()
            }
            override fun onFailure(call: Call<String>?, t: Throwable?) {
                _res.value = "FAILED"
            }
        })
    }
}

dan ini di GithubApiService.kt

private val retrofit = Retrofit.Builder()
    .addConverterFactory(GsonConverterFactory.create())
    .baseUrl("https://api.github.com/")
    .build()

interface GithubService{
    @GET("repos/laravel/laravel/contributors")
    fun showList():
            Call<String>
}

object GithubAPI{
    val retrofitService = retrofit.create(GithubService::class.java)
}

Jadi saat di OverviewFragment saya binding ke TextView mengambil dari viewModel kok dia null, kira-kira kenapa?