关于dart：Flutter应用程序错误-在null上调用了getter’value’

Flutter app error - getter 'value' was called on null

我正在构建一个应用程序，需要在"相机"视图顶部显示一些内容。这是我的代码。

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import 'dart:async';
import 'package:flutter/material.dart';
import 'package:camera/camera.dart';

List<CameraDescription> cameras;

void main() async {
runApp(new MaterialApp(
home: new CameraApp(),
));

}

class CameraApp extends StatefulWidget {
@override
_CameraAppState createState() => new _CameraAppState();
}

class _CameraAppState extends State<CameraApp> {
CameraController controller;
var cameras;
bool cameraGot = false;

Future<Null> getCamera() async {
cameras = await availableCameras();
setState(() {
this.cameras = cameras;
this.cameraGot = true;
});
}

@override
void initState() {
getCamera();
super.initState();

if(this.cameraGot) {
controller = new CameraController(this.cameras[0], ResolutionPreset.medium);
controller.initialize().then((_) {
if (!mounted) {
return;
}
setState(() {});
});
}

}

@override
void dispose() {
controller?.dispose();
super.dispose();
}

@override
Widget build(BuildContext context) {

// camera widget
Widget cameraView = new Container(
child: new Row(children: [
new Expanded(
child: new Column(
children: <Widget>[
new AspectRatio(
aspectRatio:
controller.value.aspectRatio,
child: new CameraPreview(controller)
)
]
),
)
])
);

return new Scaffold(
body: new Stack(
children: <Widget>[
!this.controller.value.initialized ? new Container() : cameraView,

// ---On top of Camera view add one mroe widget---

],
),
);
}
}

每当我构建应用程序时，都会出现以下错误...

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I/flutter ( 6911): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 6911): The following NoSuchMethodError was thrown building CameraApp(dirty, state: _CameraAppState#b6034):
I/flutter ( 6911): The getter 'value' was called on null.
I/flutter ( 6911): Receiver: null
I/flutter ( 6911): Tried calling: value

无法判断。找出错误所在。