Per https://stackoverflow.com/a/45990477/1649135 если вы помещаете виджеты, которые не имеют внутренней ширины в строке, вы должны вкладывать их в гибкий виджет.
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Welcome to Flutter',
home: new Scaffold(
appBar: new AppBar(
title: new Text('Welcome to Flutter'),
),
body: new Container(
padding: new EdgeInsets.all(20.0),
child: new Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
new Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
new Flexible(
child: new Text('User Name:'),
),
new Flexible(
child: new Container(
margin: const EdgeInsets.all(15.0),
padding: const EdgeInsets.all(3.0),
decoration: new BoxDecoration(
border: new Border.all(color: Colors.blueAccent)),
child: new TextField(
style: Theme.of(context).textTheme.body1,
),
),
),
],
),
new Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
new Flexible(
child: new Text('Password:'),
),
new Flexible(
child: new Container(
margin: const EdgeInsets.all(15.0),
padding: const EdgeInsets.all(3.0),
decoration: new BoxDecoration(
border: new Border.all(color: Colors.blueAccent)),
child: new TextField(
style: Theme.of(context).textTheme.body1,
),
),
),
],
),
],
),
)),
);
}
}