Reproducible error for qr code

qr_rep_code.dart

import 'dart:io';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:qr_code_scanner/qr_code_scanner.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: TestStart(),
    );
  }
}

class TestStart extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: TextFormField(
          decoration: InputDecoration(
            labelText: 'Search',
            prefixIcon: IconButton(
              icon: Icon(Icons.qr_code),
              onPressed: () async {
                final res = await Navigator.of(context).push(
                  MaterialPageRoute(
                    builder: (BuildContext context) => QrScannerWidget(),
                  ),
                );
                print(res);
              },
            ),
          ),
        ),
      ),
    );
  }
}

class QrScannerWidget extends StatefulWidget {
  QrScannerWidget({Key key}) : super(key: key);

  @override
  _QrScannerWidgetState createState() => _QrScannerWidgetState();
}

class _QrScannerWidgetState extends State<QrScannerWidget> {
  GlobalKey qrKey = GlobalKey();
  QRViewController controller;
  var qrText = '  Henüz tarama yapmadın';

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      body: Stack(
        children: [
          QRView(
            key: qrKey,
            overlay: QrScannerOverlayShape(
              borderRadius: 16,
              borderColor: Colors.white,
              borderLength: 120,
              borderWidth: 10,
              cutOutSize: 300,
            ),
            onQRViewCreated: _onQRViewCreate,
          ),
          Align(
            alignment: Alignment.center,
            child: Container(
              margin: EdgeInsets.all(8.0),
              constraints: BoxConstraints(maxWidth: 250),
              child: Image.asset(
                'assets/barcode.png',
                fit: BoxFit.cover,
                color: Colors.grey[300].withOpacity(0.3),
              ),
            ),
          )
        ],
      ),
    );
  }

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

  void _onQRViewCreate(QRViewController controller) {
    this.controller = controller;

    String qrData;

    controller.scannedDataStream.listen((scanData) {
      if (scanData != null) {
        print('qrdata: ${scanData.code}');

        qrData = scanData.code;
        qrData = qrData.replaceAll(new RegExp(r'[^\w\s]+'), '');
        print(qrData);

        controller.dispose();
      }
    }, onDone: () {
      Navigator.of(context).pop(qrData);
    });
  }
}