A non-null String must be provided to a Text widget.というエラー

Question

main.dart----

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
        // This makes the visual density adapt to the platform that you run
        // the app on. For desktop platforms, the controls will be smaller and
        // closer together (more dense) than on mobile platforms.
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: XDPixel2(),
    );
  }
}

class XDPixel2 extends StatefulWidget {
  XDPixel2({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<XDPixel2> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

XDPixel2.dart---

import 'package:adobe_xd/page_link.dart';

class XDPixel2 extends StatelessWidget {
  XDPixel2({
    Key key,
  }) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
      backgroundColor: const Color(0xffffffff),
      body: Stack(
        children: <Widget>[
          Transform.translate(
            offset: Offset(20.0, 157.0),
            child:
                // Adobe XD layer: 'topimg' (shape)
                Container(
              width: 375.0,
              height: 141.0,
              decoration: BoxDecoration(
                image: DecorationImage(
                  image: const AssetImage('assets/images/top.jpg'),
                  fit: BoxFit.fill,
                ),
              ),
            ),
          ),
          Transform.translate(
            offset: Offset(72.0, 374.0),
            child:
                // Adobe XD layer: '角丸長方形 5 のコピー 2' (shape)
                Container(
              width: 271.0,
              height: 49.0,
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(20.0),
                color: const Color(0xffffffff),
                border: Border.all(width: 1.0, color: const Color(0xff161616)),
              ),
            ),
          ),
          Transform.translate(
            offset: Offset(88.4, 374.4),
            child: PageLink(
              links: [
                PageLinkInfo(
                  transition: LinkTransition.Fade,
                  ease: Curves.easeOut,
                  duration: 0.3,
                  pageBuilder: () => XDPixel21(),
                ),
              ],
              child: SizedBox(
                width: 234.0,
                height: 49.0,
                child: Text(
                  'ログイン',
                  style: TextStyle(
                    fontFamily: 'NotoSansJP',
                    fontSize: 29.170000076293945,
                    color: const Color(0xff161616),
                    fontWeight: FontWeight.w300,
                    height: 1.3712718510586308,
                  ),
                  textAlign: TextAlign.center,
                ),
              ),
            ),
          ),
          Transform.translate(
            offset: Offset(72.0, 482.0),
            child:
                // Adobe XD layer: '角丸長方形 5 のコピー 3' (shape)
                Container(
              width: 271.0,
              height: 49.0,
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(20.0),
                color: const Color(0xffffffff),
                border: Border.all(width: 1.0, color: const Color(0xff161616)),
              ),
            ),
          ),
          Transform.translate(
            offset: Offset(72.4, 483.4),
            child: SizedBox(
              width: 270.0,
              height: 48.0,
              child: Text(
                'ゲスト',
                style: TextStyle(
                  fontFamily: 'NotoSansJP',
                  fontSize: 29.166669845581055,
                  color: const Color(0xff161616),
                  fontWeight: FontWeight.w300,
                  height: 1.3714284219547357,
                ),
                textAlign: TextAlign.center,
              ),
            ),
          ),
        ],
      ),
    ),
     );
  }
}

エラー

A non-null String must be provided to a Text widget.
'package:flutter/src/widgets/text.dart':
Failed assertion: line 370 pos 10: 'data != null'
The relevant error-causing widget was: 
  XDPixel2 file:///C:/flutter_kl/flutter_kl/lib/main.dart:29:13

デフォルトのmain.dartに変更を加えてアプリを起動させようとしたところ、
このようなエラーが出てしまいました。

このエラーが出るまでは起動できており、誤ってmain.dartを消してしまったので
新たな別プロジェクトを立ち上げ、そこからmain.dartを引っ張ってきたのですが適応がうまくいかず・・
どのように間違っているのでしょうか。

Answer 1

デフォルトのmain.dartはただのデモであり、記述に大して意味はないということを理解しました。

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

をXDPixel2に記述し

widget_test.dartファイルの

await tester.pumpWidget(MyApp());

を

await tester.pumpWidget(XDPixel2());

に変更することで適用でき、解決しました。