build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  return Container(
    decoration: BoxDecoration(
      color: Theme.of(context).colorScheme.surface,
      borderRadius: ROUNDED_BORDER_RADIUS,
    ),
    child: Column(
      mainAxisSize: MainAxisSize.max,
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Row(
          mainAxisSize: MainAxisSize.max,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(barcode, style: Theme.of(context).textTheme.subtitle1),
          ],
        ),
        const SizedBox(
          height: 12.0,
        ),
        const CircularProgressIndicator()
      ],
    ),
  );
}