How to fix "The Method[] was called on null" - flutter

Question

I was trying to make a class for scanning barcodes with "fast_qr_reader_view: ^0.1.5" dependencies. But when I call this class on my main.dart this error came out "NoSuchMethodError: The method[] was called on null.Receiver:null. Tried calling:"
I already tried writing on the main.dart and works, but when Im calling it from a different class doesn't work
This is my code that shows me the error main.dart
import 'package:flutter/material.dart';
import 'RegistroTrajetaOCedulaWidget.dart';
import 'package:pay_id/Screens/testScan.dart';
import 'Screens/dashboard.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(
primarySwatch: Colors.blue,
),
debugShowCheckedModeBanner: false,
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return Scan();
}
}
testScan.dart
import 'package:flutter/material.dart';
import 'dart:async';
import 'package:fast_qr_reader_view/fast_qr_reader_view.dart';
import 'package:lamp/lamp.dart';
List<CameraDescription> cameras;
Future<Null> main() async {
// Fetch the available cameras before initializing the app.
try {
cameras = await availableCameras();
} on QRReaderException catch (e) {
logError(e.code, e.description);
}
runApp(new Scan());
}
void logError(String code, String message) =>
print('Error: $code\nError Message: $message');
class Scan extends StatefulWidget {
#override
_MyAppState createState() => new _MyAppState();
}
class _MyAppState extends State<Scan> with SingleTickerProviderStateMixin {
QRReaderController controller;
final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
AnimationController animationController;
#override
void initState() {
super.initState();
animationController = new AnimationController(
vsync: this,
duration: new Duration(seconds: 3),
);
animationController.addListener(() {
this.setState(() {});
});
animationController.forward();
verticalPosition = Tween<double>(begin: 0.0, end: 300.0).animate(
CurvedAnimation(parent: animationController, curve: Curves.linear))
..addStatusListener((state) {
if (state == AnimationStatus.completed) {
animationController.reverse();
} else if (state == AnimationStatus.dismissed) {
animationController.forward();
}
});
// pick the first available camera
onNewCameraSelected(cameras[0]);
}
Animation<double> verticalPosition;
#override
Widget build(BuildContext context) {
return new Scaffold(
key: _scaffoldKey,
appBar: new AppBar(
title: const Text('Prueba escaner'),
),
floatingActionButton: FloatingActionButton(
child: new Icon(Icons.check),
onPressed: () {
Lamp.turnOn();
},
),
body: Stack(
children: <Widget>[
new Container(
child: new Padding(
padding: const EdgeInsets.all(0.0),
child: new Center(
child: _cameraPreviewWidget(),
),
),
),
Center(
child: Stack(
children: <Widget>[
SizedBox(
height: 300.0,
width: 300.0,
child: Container(
decoration: BoxDecoration(
border: Border.all(color: Colors.red, width: 2.0)),
),
),
Positioned(
top: verticalPosition.value,
child: Container(
width: 300.0,
height: 2.0,
color: Colors.red,
),
)
],
),
),
],
),
);
}
/// Display the preview from the camera (or a message if the preview is not available).
Widget _cameraPreviewWidget() {
if (controller == null || !controller.value.isInitialized) {
return const Text(
'No camera selected',
style: const TextStyle(
color: Colors.white,
fontSize: 24.0,
fontWeight: FontWeight.w900,
),
);
} else {
return new AspectRatio(
aspectRatio: controller.value.aspectRatio,
child: new QRReaderPreview(controller),
);
}
}
void onCodeRead(dynamic value) {
showInSnackBar(value.toString());
// ... do something
// wait 5 seconds then start scanning again.
new Future.delayed(const Duration(seconds: 5), controller.startScanning);
}
void onNewCameraSelected(CameraDescription cameraDescription) async {
if (controller != null) {
await controller.dispose();
}
controller = new QRReaderController(cameraDescription,
ResolutionPreset.high, [CodeFormat.qr, CodeFormat.pdf417], onCodeRead);
// If the controller is updated then update the UI.
controller.addListener(() {
if (mounted) setState(() {});
if (controller.value.hasError) {
showInSnackBar('Camera error ${controller.value.errorDescription}');
}
});
try {
await controller.initialize();
} on QRReaderException catch (e) {
logError(e.code, e.description);
showInSnackBar('Error: ${e.code}\n${e.description}');
}
if (mounted) {
setState(() {});
controller.startScanning();
}
}
void showInSnackBar(String message) {
_scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(message)));
}
}

Answer 1

cameras did not init
you can init cameras in testScan.dart directly
_asyncMethod() async {
cameras = await availableCameras();
onNewCameraSelected(cameras[0]);
print('camera ${cameras.length}');
}
#override
void initState() {
super.initState();
...
_asyncMethod();
}
demo for init cameras in testScan.dart directly
fulll test demo for testScan.dart
import 'package:flutter/material.dart';
import 'dart:async';
import 'package:fast_qr_reader_view/fast_qr_reader_view.dart';
import 'package:lamp/lamp.dart';
List<CameraDescription> cameras;
/*Future<Null> main() async {
// Fetch the available cameras before initializing the app.
try {
cameras = await availableCameras();
} on QRReaderException catch (e) {
logError(e.code, e.description);
}
runApp(new Scan());
}*/
void logError(String code, String message) =>
print('Error: $code\nError Message: $message');
class Scan extends StatefulWidget {
#override
_ScanState createState() => new _ScanState();
}
class _ScanState extends State<Scan> with SingleTickerProviderStateMixin {
QRReaderController controller;
final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
AnimationController animationController;
_asyncMethod() async {
cameras = await availableCameras();
onNewCameraSelected(cameras[0]);
print('camera ${cameras.length}');
}
#override
void initState() {
super.initState();
animationController = new AnimationController(
vsync: this,
duration: new Duration(seconds: 3),
);
animationController.addListener(() {
this.setState(() {});
});
animationController.forward();
verticalPosition = Tween<double>(begin: 0.0, end: 300.0).animate(
CurvedAnimation(parent: animationController, curve: Curves.linear))
..addStatusListener((state) {
if (state == AnimationStatus.completed) {
animationController.reverse();
} else if (state == AnimationStatus.dismissed) {
animationController.forward();
}
});
// pick the first available camera
_asyncMethod();
}
Animation<double> verticalPosition;
#override
Widget build(BuildContext context) {
return new Scaffold(
key: _scaffoldKey,
appBar: new AppBar(
title: const Text('Prueba escaner'),
),
floatingActionButton: FloatingActionButton(
child: new Icon(Icons.check),
onPressed: () {
Lamp.turnOn();
},
),
body: Stack(
children: <Widget>[
new Container(
child: new Padding(
padding: const EdgeInsets.all(0.0),
child: new Center(
child: _cameraPreviewWidget(),
),
),
),
Center(
child: Stack(
children: <Widget>[
SizedBox(
height: 300.0,
width: 300.0,
child: Container(
decoration: BoxDecoration(
border: Border.all(color: Colors.red, width: 2.0)),
),
),
Positioned(
top: verticalPosition.value,
child: Container(
width: 300.0,
height: 2.0,
color: Colors.red,
),
)
],
),
),
],
),
);
}
/// Display the preview from the camera (or a message if the preview is not available).
Widget _cameraPreviewWidget() {
if (controller == null || !controller.value.isInitialized) {
return const Text(
'No camera selected',
style: const TextStyle(
color: Colors.white,
fontSize: 24.0,
fontWeight: FontWeight.w900,
),
);
} else {
return new AspectRatio(
aspectRatio: controller.value.aspectRatio,
child: new QRReaderPreview(controller),
);
}
}
void onCodeRead(dynamic value) {
showInSnackBar(value.toString());
// ... do something
// wait 5 seconds then start scanning again.
new Future.delayed(const Duration(seconds: 5), controller.startScanning);
}
void onNewCameraSelected(CameraDescription cameraDescription) async {
if (controller != null) {
await controller.dispose();
}
controller = new QRReaderController(cameraDescription,
ResolutionPreset.high, [CodeFormat.qr, CodeFormat.pdf417], onCodeRead);
// If the controller is updated then update the UI.
controller.addListener(() {
if (mounted) setState(() {});
if (controller.value.hasError) {
showInSnackBar('Camera error ${controller.value.errorDescription}');
}
});
try {
await controller.initialize();
} on QRReaderException catch (e) {
logError(e.code, e.description);
showInSnackBar('Error: ${e.code}\n${e.description}');
}
if (mounted) {
setState(() {});
controller.startScanning();
}
}
void showInSnackBar(String message) {
_scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(message)));
}
}