Flutter Charts长度获取方法‘NoSuchMethodError’在null上调用

import 'package:flutter_charts/flutter_charts.dart';

Chart chart = Chart();

import 'package:flutter/material.dart';
import 'package:flutter_charts/flutter_charts.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    Chart chart = Chart(
      data: [
        [1, 2, 3, 4, 5], // 第一个数据系列
        [6, 7, 8, 9, 10], // 第二个数据系列
      ],
    );

    int seriesCount = chart.data.length;
    int seriesLength = chart.data[0].length;

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Charts Length Example'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text('Series Count: $seriesCount'),
              Text('Series Length: $seriesLength'),
            ],
          ),
        ),
      ),
    );
  }
}