Попытка вызвать виртуальный метод void androidx.lifecycle.LiveData.observe для ссылки на нулевой объект

Question

Я пытаюсь использовать mvvm в первый раз и закончил с ошибкой ссылки на нулевой объект.

Я ссылался на эту ссылку для того же: https://medium.com/@kenkyee / retrofit2-doesnt- handle-empty-content-answers-bef2b33ee8ea

Я получаю ответ от api и сохраняю его в базе данных комнаты, а затем извлекаю то же самое с использованием модели представления и наблюдателя из базы данных комнаты для отображения в списке.

Вот мой код:

Репозиторий:

public class WebServiceRepository {
    String responseString = "";
    Application application;
    public  WebServiceRepository(Application application){
        this.application = application;
    }
    private static OkHttpClient providesOkHttpClientBuilder(){

        OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
        return httpClient.readTimeout(1200, TimeUnit.SECONDS)
                .connectTimeout(1200, TimeUnit.SECONDS).build();

    }


    List<Photos> webserviceResponseList = new ArrayList<>();

    public LiveData<List<Photos>> providesWebService() {

        final MutableLiveData<List<Photos>> data = new MutableLiveData<>();


        try {
            Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl(APIUrl.BASE_URL)
         /*           .addConverterFactory(new NullOnEmptyConverterFactory())
                    .addConverterFactory(ScalarsConverterFactory.create())*/
                    .addConverterFactory(GsonConverterFactory.create())
                    .client(providesOkHttpClientBuilder())
                    .build();

            //Defining retrofit api service
            APIService service = retrofit.create(APIService.class);

            service.makeRequest().enqueue(new Callback<String>() {
                @Override
                public void onResponse(Call<String> call, Response<String> response) {

                    responseString = response.toString();
                    Log.d("Repository","Response::::"+response.body());
                    webserviceResponseList = parseJson(response.body());
                    PhotosRoomDBRepository photosRoomDBRepository =
                            new PhotosRoomDBRepository(application);
                    photosRoomDBRepository.insertPhotos(webserviceResponseList);

                    data.setValue(webserviceResponseList);
                }

                @Override
                public void onFailure(Call<String> call, Throwable t) {
                    Log.d("Repository","Failed:::");
                }
            });


/*
         service.makeRequest().enqueue(new Callback<String>() {
             @Override public void onResponse(
                     Call<String> call, Response<String> response
             ) {

                 Log.d("Repository","Response::::"+response.body());
                 webserviceResponseList = parseJson(response.body());
                 PhotosRoomDBRepository photosRoomDBRepository =
                         new PhotosRoomDBRepository(application);
                 photosRoomDBRepository.insertPosts(webserviceResponseList);

                 data.setValue(webserviceResponseList);

             }

             @Override public void onFailure(Call<String> call, Throwable t) {

                 Log.d("Repository","Failed:::");
             }
         });*/

        }catch (Exception e){
            e.printStackTrace();
        }

        return  data;

    }


    private List<Photos> parseJson(String response) {

        List<Photos> apiResults = new ArrayList<>();

        JSONObject jsonObject;

        JSONArray jsonArray;

        try {
            jsonArray = new JSONArray(response);

            for (int i = 0; i < jsonArray.length(); i++) {
                JSONObject object = jsonArray.getJSONObject(i);

                Photos photos = new Photos();

                photos.setId(Integer.parseInt(object.getString("id")));
                photos.setAlbumId(Integer.parseInt(object.getString("albumId")));
                photos.setTitle(object.getString("title"));
                photos.setUrl(object.getString("url"));
                photos.setThumbnailUrl(object.getString("thumbnailUrl"));

                apiResults.add(photos);
            }


        } catch (JSONException e) {
            e.printStackTrace();
        }

        Log.i(getClass().getSimpleName(), String.valueOf(apiResults.size()));
        return apiResults;

    }
}

Просмотр модели

public class MainViewModel extends AndroidViewModel {
    // TODO: Implement the ViewModel

    private PhotosRoomDBRepository photosRoomDBRepository;
    private LiveData<List<Photos>> mPhotos;
    WebServiceRepository webServiceRepository ;
    private LiveData<List<Photos>>  photoObservable;

    public MainViewModel(Application application){
        super(application);

        photosRoomDBRepository = new PhotosRoomDBRepository(application);
        webServiceRepository = new WebServiceRepository(application);

        photoObservable = webServiceRepository.providesWebService();
        //postRoomDBRepository.insertPosts(retroObservable.getValue());
        photoObservable = photosRoomDBRepository.getAllPhotos();
    }

    public LiveData<List<Photos>> getAllPhotos() {
        return mPhotos;
    }

}

Фрагмент:

public class MainFragment extends Fragment {

    private MainViewModel mViewModel;

    ArrayList<Photos> photosArrayList = new ArrayList<>();
    private RecyclerView recyclerView;
    private PhotosAdapter photosAdapter;

    ProgressDialog progressDialog;

    public static MainFragment newInstance() {
        return new MainFragment();
    }

    @Nullable
    @Override
    public View onCreateView(
            @NonNull LayoutInflater inflater, @Nullable ViewGroup container,
            @Nullable Bundle savedInstanceState
    ) {
        View view = inflater.inflate(R.layout.main_fragment, container, false);

        setUpUI(view);

        return view;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mViewModel = ViewModelProviders.of(getActivity()).get(MainViewModel.class);
        // TODO: Use the ViewModel

        setUpRecyclerView();

        getDataFromLocalDatabase();
    }

    public void setUpUI(View view) {

        recyclerView = view.findViewById(R.id.recycler_view_photos);
    }

    public void setUpRecyclerView() {

        photosAdapter = new PhotosAdapter(photosArrayList, getActivity());
        RecyclerView.LayoutManager mLayoutManager =
                new GridLayoutManager(getActivity(), 3);
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        recyclerView.addItemDecoration(new GridSpacingItemDecoration(3, 10,
                true, 0
        ));

        recyclerView.setAdapter(photosAdapter);

    }

    public void getDataFromLocalDatabase(){

        progressDialog = ProgressDialog.show(getActivity(), "Loading...",
                "Please wait...", true);

        mViewModel.getAllPhotos().observe(getActivity(),
                new Observer<List<Photos>>() {
            @Override
            public void onChanged(@Nullable List<Photos> photos) {
                photosAdapter.updateList(photos);
                if(progressDialog!=null && progressDialog.isShowing()){
                    progressDialog.dismiss();
                }
            }
        });
    }
}

Я получаю сообщение об ошибке:

   mViewModel.getAllPhotos().observe(getActivity(),
            new Observer<List<Photos>>() {
        @Override
        public void onChanged(@Nullable List<Photos> photos) {
            photosAdapter.updateList(photos);
            if(progressDialog!=null && progressDialog.isShowing()){
                progressDialog.dismiss();
            }
        }
    });

Помогите, пожалуйста, с тем же спасибо ..