A Flutter example of how to make a Widget member value have a default value, while being extension/compose and null safe. Part of this Twitter discussion: https://twitter.com/RydMike/status/1243200280809930753

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: Scaffold(        
        body: SuperWidget(
          child: Text('This is supposed to have a red background'),
        ),
      ),
    ),
  );
}

// -----------------------------------------------------------------------------
// This would be in your "super_widget.dart" file if you want to keep it separate
/// A widget that uses [SomeBaseWidget] but still expose some customization
// As is, it is useless – it's just for the sake of example
class SuperWidget extends SomeBaseWidget {
  const SuperWidget({Key key, this.color, this.child}) : super(key: key);
  final Color color;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return SomeBaseWidget(
      // NOTE: This color has default value, is extension and null safe
      color: color,
      child: child,
    );
  }
}

// -----------------------------------------------------------------------------
// This would be in your "some_base_widget.dart" file
/// [SomeBaseWidget] defaults to red color background
const _kDefaultColor = Colors.red;

/// A customizable low level widget with some default value
class SomeBaseWidget extends StatelessWidget {
  const SomeBaseWidget({
    Key key,
    this.color = _kDefaultColor,
    this.child,
  }) : super(key: key);

  /// Color of the [SomeBaseWidget]
  final Color color;
  /// The child of [SomeBaseWidget]
  final Widget child;

  @override
  Widget build(BuildContext context) {    
    // If null is passed we set it back to the default value, this is now an
    // implementation null safety and extension/composition safety detail
    final color = this.color ?? _kDefaultColor;
    
    return Container(
      color: color,
      alignment: Alignment.center,
      child: child,
    );
  }
}