movie_repository.dart

// Before
Future<List<ContentDetailData>> getPopularMovies(int page) =>
      _movieApi.getPopular(page).then((content) => _getMovies(content, true));

  Future<List<ContentDetailData>> _getMovies(
      PageContent content, bool arePopular) async {
      // save, map and perform other operations   
  }
  
// After  
Future<List<ContentDetailData>> getPopularMovies(int page) =>
      _movieApi.getPopular(page).then((content) {
        if (content is ContentListResponse) {
          return _getMovies(content, true);
        }
        return <ContentDetailData>[];
      });