颤振错误：列的子级不得包含任何空值，但在索引 0 处找到空值

Question

我在android studio中创建了一个应用程序，用于从一个屏幕导航到另一个屏幕。在这里，两个无状态小部件被创建为两个屏幕，并且都包含一个按钮来导航页面。然而，当我运行应用程序时，我的android手机上会生成一个红色屏幕，我会收到一个错误，说

exception 'Column's children must not contain any null values, but a null value was found at index 0'.

我提供了我的代码如下：

第一个屏幕

class FirstScreen extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("First Screen"),
),
body: Container(
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
center(
decoration: new BoxDecoration(
image: new DecorationImage(
image: new AssetImage('assets/new 7wonders.jpg'),
fit: BoxFit.cover,
),
),
),
Text('New 7 Wonders',
style: TextStyle(fontSize: 40, fontStyle: FontStyle.italic),
),
RaisedButton(
child: Text("Bang Here"),
onPressed: (){
Navigator.push(context, MaterialPageRoute(builder: (context) => SecondScreen()));
},
color: Colors.red,
textColor: Colors.yellow,
padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
splashColor: Colors.grey,
)
],
),
),
),
);
}
center({BoxDecoration decoration}) {}
}

第二个屏幕

class SecondScreen extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Second Screen"),
),
body: RaisedButton(
child: Text("Go to First page"),
onPressed:() {
Navigator.pop(context);
},
),
);
}
}

Answer 1

您的center方法应该返回一个Widget，它目前正在向Column提供null。

改为：

Widget center() {
// return a decorated box widget which gives you the decoration property
return Image(
image: AssetImage(
'assets/new 7wonders.jpg',),
fit: BoxFit.cover,
);
}
}

然后在你的Column中使用，比如：

Container(
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
// call the center method which returns a Widget
center(),
Text(
'New 7 Wonders',
style: TextStyle(fontSize: 40, fontStyle: FontStyle.italic),
),
RaisedButton(
child: Text("Bang Here"),
onPressed: () {
Navigator.push(context,
MaterialPageRoute(builder: (context) => SecondScreen()));
},
color: Colors.red,
textColor: Colors.yellow,
padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
splashColor: Colors.grey,
)
],
),
),
),

Answer 2

您必须返回中心的任何小部件

center({BoxDecoration decoration}) {
return Container();
}

Answer 3

您尝试在第24行写入Center而不是center？并且在中心必须是将返回例如Containter()

Answer 4

在第24行中，您返回了null值。您可以像这样实现center方法；

return Container();

Answer 5

Remove center use this
Container(
height: 100,       // height and width according to your ui
width:100,
child:Image.asset(('assets/new7wonders.jpg',fit: BoxFit.cover,), // use for local image from asset and please change image name in code as well as in asset folder.there should  not be space between image name .

),