build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  return Container(
    padding: const EdgeInsets.symmetric(horizontal: 20.0),
    margin: const EdgeInsets.only(top: 20.0, bottom: 24.0),
    child: Text(
      title,
      style: Theme.of(context).textTheme.headline1,
    ),
  );
}