title: Flutter default value for a Function
body: <p>I am currently learning a course on Flutter. They provide you with an old stub project (nullable).
  When I try to migrate to Flutter 2.12.0 or higher - null safety kicks in.
  I have a basic understanding on how it works - but this I cannot find anywhere on Google or StackOverFlow.</p>
  <p>I have a custom Card widget. It requires a Color parameter. Also I need it to be able to receive a Widget child parameter as well as an onPress Function, but I don't want to make them required.
  Please, help. How do I create a default value for Widgets and Functions?</p>
  <pre><code>class ReusableCard extends StatelessWidget {
    ReusableCard({@required this.cardColor, this.cardChild, this.onPress});
    final Color cardColor;
    final Widget cardChild;
    final Function onPress;
  
    @override
    Widget build(BuildContext context) {
      return GestureDetector(
        onTap: onPress,
        child: Container(
          child: cardChild,
          margin: EdgeInsets.all(10.0),
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(10.0),
            color: cardColor,
          ),
        ),
      );
    }
  }
  </code></pre>