list - 用于生成列表的错误“在 null 上调用了方法‘调用’”

Question

在我的应用程序中，我使用以下代码：

@override
  Widget build(BuildContext context) {
      List<double> listMax = List.from(data_snap()['history']);
      final listMaxreversed = List.from(listMax.reversed);
      final list_one_week = listMaxreversed.take(
          data_snap()['history']['1w']);
      final list_one_week_list = List.from(list_one_week);
      final list_one_week_list_rev = List.from(list_one_week_list.reversed);
      List<FlSpot> spots_one_week =
      list_one_week_list_rev
          .asMap()
          .entries
          .map((e) {
        return FlSpot(e.key.toDouble(), e.value);
      }).toList();
      print(spots_one_week);

return Scaffold(
  key: _scaffoldKey,
  appBar: AppBar(
    iconTheme: IconThemeData(color: Colors.lightGreen),
    backgroundColor: Colors.grey[100],
    title: const Text('Details', style:
    TextStyle(color: Colors.lightGreen)),
  ),
  body: Padding(
   //some code here...
                  Row(
                      children: [ _loading ? CircularProgressIndicator():
                      new Container(
                        child:
                        LineChart(
                          LineChartData(
                            lineTouchData: LineTouchData(enabled: true),
                            gridData: FlGridData(
                              show: false,
                              drawHorizontalLine: false,
                              drawVerticalLine: false,
                              getDrawingVerticalLine: (value) {
                                return FlLine(
                                  color: Colors.black,
                                  strokeWidth: 1,
                                );
                              },
                            ),
                            lineBarsData: [
                              LineChartBarData(
                                  spots: spots_one_week, //inserts the spots here

片刻之后，我的手机出现以下错误。此后，图表将正确显示。

The method 'call' was called on null.
Receiver: null
Tried calling: call()

我想我必须包括类似异步任务的东西。当我定义listMax时直接发生错误。我该如何解决这个问题？