With material

main.dart

import 'package:flutter/material.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(const Test());
}

class Test extends StatefulWidget {
  const Test({Key? key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => _TestState();
}

class _TestState extends State<Test> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: const ExampleWidget(),
      builder: (context, child) {
        return Scaffold(
          body: Column(
            children: [
              Container(height: 40, color: Colors.red),
              Expanded(child: child!),
            ],
          ),
        );
      },
    );
  }
}

class ExampleWidget extends StatelessWidget {
  const ExampleWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.blue,
    );
  }
}