问参数类型'Future<Uint8List?>‘不能分配给参数类型'Future<Uint8List>’？

EN

The argument type 'Future<Uint8List?>' can't be assigned to the parameter type 'Future<Uint8List>?
The return type 'Uint8List?' isn't a 'FutureOr<Uint8List>', as required by the closure's context.
The argument type 'List<int>?' can't be assigned to the parameter type 'List<int>'.

class _MyHomePageState extends State<MyHomePage> {
  String _url =
      "https://www.au-sonpo.co.jp/corporate/upload/article/89/article_89_1.pdf";

  void _refresh() {
    setState(() {
      // other _url
      _url =
      "https://www.au-sonpo.co.jp/corporate/upload/article/90/article_90_1.pdf";
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: FutureBuilder<Uint8List>(
        future:  _fetchPdfContent(_url),
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return PdfPreview(
              allowPrinting: false,
              allowSharing: false,
              canChangePageFormat: false,
              initialPageFormat:
              PdfPageFormat(100 * PdfPageFormat.mm, 120 * PdfPageFormat.mm),
              build: (format)  =>  snapshot.data,
            );
          }
          return Center(
            child: CircularProgressIndicator(),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _refresh,
        tooltip: 'Refesh',
        child: Icon(Icons.refresh),
      ),
    );
  }

  Future<Uint8List?> _fetchPdfContent(final String url) async {
    try {
      final Response<List<int>> response = await Dio().get<List<int>>(
        url,
        options: Options(responseType: ResponseType.bytes),
      );
      return Uint8List.fromList(response.data);
    } catch (e) {
      print(e);
      return null;
    }
  }
}