add back buttons to pages; fix error builder for window page & zoom slider page

@@ -50,6 +50,7 @@ class _BarcodeListScannerWithControllerState
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('With ValueListenableBuilder')),
       backgroundColor: Colors.black,
       body: Builder(
         builder: (context) {

@@ -50,6 +50,7 @@ class _BarcodeScannerWithControllerState
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('With controller')),
       backgroundColor: Colors.black,
       body: Builder(
         builder: (context) {

@@ -70,6 +70,7 @@ class _BarcodeScannerPageViewState extends State<BarcodeScannerPageView>
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('With PageView')),
       backgroundColor: Colors.black,
       body: PageView(
         children: [

@@ -52,6 +52,7 @@ class _BarcodeScannerReturningImageState
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('Returning image')),
       body: SafeArea(
         child: Column(
           children: [

@@ -3,6 +3,8 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:mobile_scanner/mobile_scanner.dart';
 
+import 'package:mobile_scanner_example/scanner_error_widget.dart';
+
 class BarcodeScannerWithScanWindow extends StatefulWidget {
   const BarcodeScannerWithScanWindow({Key? key}) : super(key: key);
 
@@ -32,6 +34,7 @@ class _BarcodeScannerWithScanWindowState
       height: 200,
     );
     return Scaffold(
+      appBar: AppBar(title: const Text('With Scan window')),
       backgroundColor: Colors.black,
       body: Builder(
         builder: (context) {
@@ -47,6 +50,9 @@ class _BarcodeScannerWithScanWindowState
                     this.arguments = arguments;
                   });
                 },
+                errorBuilder: (context, error, child) {
+                  return ScannerErrorWidget(error: error);
+                },
                 onDetect: onDetect,
               ),
               if (barcode != null &&

@@ -18,6 +18,7 @@ class _BarcodeScannerWithoutControllerState
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('Without controller')),
       backgroundColor: Colors.black,
       body: Builder(
         builder: (context) {

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:image_picker/image_picker.dart';
 import 'package:mobile_scanner/mobile_scanner.dart';
 
+import 'package:mobile_scanner_example/scanner_error_widget.dart';
+
 class BarcodeScannerWithZoom extends StatefulWidget {
   const BarcodeScannerWithZoom({Key? key}) : super(key: key);
 
@@ -23,6 +25,7 @@ class _BarcodeScannerWithZoomState extends State<BarcodeScannerWithZoom>
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: AppBar(title: const Text('With zoom slider')),
       backgroundColor: Colors.black,
       body: Builder(
         builder: (context) {
@@ -31,6 +34,9 @@ class _BarcodeScannerWithZoomState extends State<BarcodeScannerWithZoom>
               MobileScanner(
                 controller: controller,
                 fit: BoxFit.contain,
+                errorBuilder: (context, error, child) {
+                  return ScannerErrorWidget(error: error);
+                },
                 onDetect: (barcode) {
                   setState(() {
                     this.barcode = barcode;