Flutter2.0参数类型“颜色？”不能分配给参数类型“颜色”

在我将 flutter sdk 更新为 >=2.12.0 <3.0.0 后，出现了一个奇怪的错误，说The argument type 'Color?' can't be assigned to the parameter type 'Color'当我尝试为卡片小部件分配边框颜色时，这里发生了什么？

Card(
  shape: RoundedRectangleBorder(
    side: BorderSide(color: Colors.blue[300], width: 2.0),
    borderRadius: BorderRadius.circular(15.0)
  ),
  child: Text('Demo')),

重现错误的完整代码：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Card(
              shape: RoundedRectangleBorder(
                side: BorderSide(color: Colors.blue[300], width: 2.0),
                borderRadius: BorderRadius.circular(15.0),
              ),
              child: Text('Demo')),
        ),
      ),
    );
  }
}

回答

 color: (Colors.blue[300])!,