Flutter 2.0 The argument type 'Color?' can't be assigned to the parameter type 'Color'

user_14085125 0
Viewed 16737

After I updated the flutter sdk to >=2.12.0 <3.0.0, there's a weird error saying that The argument type 'Color?' can't be assigned to the parameter type 'Color' when I try to assign the border color to the card widget, what is going on here?

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

Full code to reproduce the error:

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')),
        ),
      ),
    );
  }
}
flutter
3 Answers
user_151162290
user_164589180

Maybe in the future using the bang operator [!]

color: Colors.blue[300]!;

will provoke some problems, if you ever get an error, it might appear the error of: Null check operator used on a null value so to solve that, you should do:

var color = Colors.blue[300];
color: color as Color

try to avoid the bang operator unless you really know what you're doing, hope it helps to someone!

user_122243080
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