Faça como eu fiz: o retorno do Widget

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

void main() {
  runApp(const MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          leading: Icon(Icons.add_task),
          title: Text('Flutter: Primeiros Passos'),
        ),
        body: ListView(
          children: const [
            Quadro(Colors.pink, Colors.purple, Colors.blueAccent, Colors.white,
                Colors.lightBlueAccent, Colors.blueGrey, 100, 140),
            Quadro(Colors.pinkAccent, Colors.amber, Colors.lightBlue,
                Colors.black, Colors.amberAccent, Colors.purple, 100, 140),
            Quadro(Colors.purple, Colors.white, Colors.green, Colors.black,
                Colors.greenAccent, Colors.limeAccent, 100, 140),
            Quadro(Colors.pink, Colors.purple, Colors.blueAccent, Colors.white,
                Colors.lightBlueAccent, Colors.blueGrey, 100, 140),
            Quadro(Colors.pinkAccent, Colors.amber, Colors.lightBlue,
                Colors.black, Colors.amberAccent, Colors.purple, 100, 140),
            Quadro(Colors.purple, Colors.white, Colors.green, Colors.black,
                Colors.greenAccent, Colors.limeAccent, 100, 140),
            SizedBox(
              height: 100,
            ),
          ],
        ),
      ),
    );
  }
}

class Quadro extends StatelessWidget {
  final Color cor1;
  final Color cor2;
  final Color cor3;
  final Color cor4;
  final Color cor5;
  final Color cor6;
  final double largura;
  final double altura;

  const Quadro(this.cor1, this.cor2, this.cor3, this.cor4, this.cor5, this.cor6,
      this.largura, this.altura,
      {Key? key})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.only(top: 10),
      child: Container(
        height: altura,
        child: ListView(
          scrollDirection: Axis.horizontal,
          children: [
            Container(
              child: Icon(
                Icons.account_circle_rounded,
                size: 50,
                color: Colors.black26,
              ),
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor1,
              ),
              height: altura,
              width: largura,
            ),
            Container(
              child: Image.network(
                'https://mdevelopers.com/storage/0_flutterheader_0c3ac92d.png',
              ),
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor2,
              ),
              height: altura,
              width: largura,
            ),
            Container(
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor3,
              ),
              height: altura,
              width: largura,
            ),
            Container(
              child: Text('Flutter',
                  textAlign: TextAlign.center,
                  style: TextStyle(
                      color: Colors.black,
                      fontSize: 20,
                      backgroundColor: Colors.blue)),
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor4,
              ),
              height: altura,
              width: largura,
            ),
            Container(
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor5,
              ),
              height: altura,
              width: largura,
            ),
            Container(
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(4),
                border: Border.all(width: 5, color: Colors.black26),
                color: cor6,
              ),
              height: altura,
              width: largura,
            )
          ],
        ),
      ),
    );
  }
}