title: Flutter 2.0 The argument type 'Color?' can't be assigned to the parameter type 'Color'
body: <p>After I updated the flutter sdk to &gt;=2.12.0 &lt;3.0.0, there's a weird error saying that <code>The argument type 'Color?' can't be assigned to the parameter type 'Color'</code> when I try to assign the border color to the card widget, what is going on here?</p>
  <pre><code>Card(
    shape: RoundedRectangleBorder(
      side: BorderSide(color: Colors.blue[300], width: 2.0),
      borderRadius: BorderRadius.circular(15.0)
    ),
    child: Text('Demo')),
  </code></pre>
  <p>Full code to reproduce the error:</p>
  <pre><code>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')),
          ),
        ),
      );
    }
  }
  </code></pre>