build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  final AppLocalizations appLocalizations = AppLocalizations.of(context);
  final TextStyle headlineStyle =
      Theme.of(context).textTheme.headline2!.apply(color: Colors.white);
  final TextStyle bodyTextStyle =
      Theme.of(context).textTheme.bodyText1!.apply(color: Colors.white);
  // Side padding is 8% of total width.
  final double sidePadding = MediaQuery.of(context).size.width * .08;

  return Scaffold(
    body: Column(children: <Widget>[
      Expanded(
        child: Padding(
          padding: EdgeInsets.symmetric(
            horizontal: sidePadding,
          ),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.spaceBetween,
            children: <Widget>[
              const Spacer(flex: 1),
              Flexible(
                flex: 4,
                child: Text(
                  appLocalizations.whatIsOff,
                  style: headlineStyle,
                ),
              ),
              Flexible(
                flex: 3,
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  mainAxisSize: MainAxisSize.min,
                  children: <Widget>[
                    Padding(
                      padding: const EdgeInsetsDirectional.only(
                        start: SMALL_SPACE,
                      ),
                      child: Text(
                        appLocalizations.country_chooser_label,
                        style: bodyTextStyle,
                      ),
                    ),
                    Container(
                      decoration: BoxDecoration(
                        border: Border.all(
                          color: Colors.white,
                          width: 1,
                        ),
                        borderRadius: BorderRadius.circular(
                          LARGE_SPACE,
                        ),
                        color: Theme.of(context).cardColor,
                      ),
                      margin:
                          const EdgeInsets.symmetric(vertical: MEDIUM_SPACE),
                      child: CountrySelector(
                        initialCountryCode: WidgetsBinding
                            .instance.window.locale.countryCode
                            ?.toLowerCase(),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsetsDirectional.only(
                        start: SMALL_SPACE,
                        bottom: VERY_SMALL_SPACE,
                      ),
                      child: Text(
                        appLocalizations.country_selection_explanation,
                        style: bodyTextStyle,
                      ),
                    ),
                  ],
                ),
              ),
            ],
          ),
        ),
      ),
      const Align(
        alignment: Alignment.bottomCenter,
        child: NextButton(OnboardingPage.WELCOME),
      ),
    ]),
    backgroundColor: Theme.of(context).appBarTheme.backgroundColor,
  );
}