java中showdialog返回值_Flutter：showDialog：build函数返回null

我有一个StatefulWidget . 然后，当我单击一个按钮时，它会显示一个警告对话框 . 当我实施时：

onTap: () {

showDialog(

context: context,

builder: (BuildContext context) {

return AlertDialog(

title: Text("Hello"),

);

}

}

一切正常 . 但是当我将构建器中的内容转移到另一个StatefulWidget时，会发生以下错误：

A build function returned null.

I/flutter ( 3647): The offending widget is: Builder

I/flutter ( 3647): Build functions must never return null. To return an empty space that causes the building widget to

I/flutter ( 3647): fill available room, return "new Container()". To return an empty space that takes as little room as

I/flutter ( 3647): possible, return "new Container(width: 0.0, height: 0.0)".

这是代码：

Here is the calling StatefulWidget:

onTap: () {

showDialog(

context: context,

builder: (BuildContext context) {

LastVacDialog(

currentDose: currDose,

currentDate: currDate,

currentIndex: i,

setValue: changeDoseValueAndDate,

);

},

);

},

Here is the new StatefulWidget:

class LastVacDialog extends StatefulWidget {

LastVacDialog({

this.currentDose,

this.currentDate,

this.setValue,

this.currentIndex,

});

final int currentDose;

final DateTime currentDate;

final void Function(int, DateTime, int) setValue;

final currentIndex;

@override

LastVacDialogState createState() => new LastVacDialogState();

}

class LastVacDialogState extends State {

int _dose;

DateTime _today;

@override

Widget build(BuildContext context) {

return AlertDialog(

title: Text("Last Dose"),

);

}

}

我的代码有问题吗？为简单起见，我省略了一些变量 .