public interface ApiService {
    @GET("movies?")
    Single<MovieResponse> getMovieList(@Query("page") int page);
}

public class ServerDataSource implements DataSource {

    @Inject
    ApiService apiServices;

    public ServerDataSource(ApiService apiServices) {
        this.apiServices = apiServices;
    }

    @Override
    public Single<MovieResponse> getMovieListResponse(int page) {
        return apiServices.getMovieList(page);
    }
}

@Module
public class ServerModule {

    @Provides
    @Singleton
    Retrofit provideCall() {
        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .readTimeout(60, TimeUnit.SECONDS)
                .writeTimeout(60, TimeUnit.SECONDS)
                .connectTimeout(60, TimeUnit.SECONDS)
                .build();

        return new Retrofit.Builder()
                .baseUrl(AppConstant.BASE_URL)
                .client(okHttpClient)
                .addConverterFactory(GsonConverterFactory.create())
                .addConverterFactory(ScalarsConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();
    }

    @Provides
    @Singleton
    ApiService provideApiServices(Retrofit retrofit) {
        return retrofit.create(ApiService.class);
    }

    @Provides
    @Singleton
    ServerDataSource provideServerDataSource(ApiService apiServices) {
        return new ServerDataSource(apiServices);
    }
}

public class HomePresenter implements HomeContract.Presenter {

    private HomeContract.View view;
    private DataSource dataSource;
    private CompositeDisposable disposable = new CompositeDisposable();

    public HomePresenter(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    @Override
    public void getMoviesList(int page) {
        view.showLoader();
        dataSource.getMovieListResponse(page).subscribeOn(Schedulers.newThread())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new SingleObserver<MovieResponse>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        disposable.add(d);
                    }

                    @Override
                    public void onSuccess(MovieResponse newsResponse) {
                        view.hideLoader();
                        view.showMoviesList(newsResponse);
                    }

                    @Override
                    public void onError(Throwable e) {
                        view.hideLoader();
                        view.showError(e.toString());
                    }
                });
    }

    @Override
    public void attachView(HomeContract.View view) {
        this.view = view;
    }

    @Override
    public void detachView() {
        this.view = null;
        if (disposable != null && disposable.size() > 0) {
            disposable.clear();
        }
    }
}

public class HomeActivity extends BaseActivity implements HomeContract.View {

    @BindView(R.id.baseFragment_loader)
    ProgressBar baseFragment_loader;
    @BindView(R.id.baseFragment_content)
    RelativeLayout baseFragment_content;
    @BindView(R.id.baseFragment_contentNewsList)
    RecyclerView baseFragment_contentNewsList;

    private HomeContract.Presenter presenter;
    private List<Datum> todayList = new ArrayList<>();
    private HomeMoviesListAdapter newsAdapter;
    @Inject
    ApiService apiService;


    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        presenter = new HomePresenter(new ServerDataSource(apiService));
    }

    @Override
    public void onStart() {
        super.onStart();
        presenter.attachView(this);
        setupViews();
    }

    @Override
    public void onStop() {
        super.onStop();
        presenter.detachView();
    }

    @Override
    public Context getViewContext() {
        return this;
    }

    @Override
    public void setupViews() {
        newsAdapter = new HomeMoviesListAdapter(todayList);
        AppConstant.setupRecyclerView_linear(getViewContext(), baseFragment_contentNewsList, newsAdapter);
        presenter.getMoviesList(1);
    }

    @Override
    public int getLayout() {
        return R.layout.activity_main;
    }

    @Override
    public void showMoviesList(MovieResponse newsList) {
        todayList.clear();
        todayList.addAll(newsList.getData());
        newsAdapter.notifyDataSetChanged();
    }

    @Override
    public void showError(String err) {
        Toast.makeText(getViewContext(), err, Toast.LENGTH_SHORT).show();
    }

    @Override
    public void showLoader() {
        baseFragment_loader.setVisibility(View.VISIBLE);
        baseFragment_content.setVisibility(View.GONE);
    }

    @Override
    public void hideLoader() {
        baseFragment_loader.setVisibility(View.GONE);
        baseFragment_content.setVisibility(View.VISIBLE);
    }
}

Caused by: java.lang.NullPointerException
    at com.example.mohammad.movietestproject.Data.Network.ServerDataSource.getMovieListResponse(ServerDataSource.java:28)
    at com.example.mohammad.movietestproject.Home.HomePresenter.getMoviesList(HomePresenter.java:25)
    at com.example.mohammad.movietestproject.Home.HomeActivity.setupViews(HomeActivity.java:72)
    at com.example.mohammad.movietestproject.Home.HomeActivity.onStart(HomeActivity.java:54)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1188)
    at android.app.Activity.performStart(Activity.java:5241)

@Override
public Single<MovieResponse> getMovieListResponse(int page) {
    return apiServices.getMovieList(page);
}