Trying to do a shpping cart project in flutter error is : The method '+' can't be unconditionally invoked because the receiver can be 'null'

import 'dart:io';

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

void main() {
  FlutterError.onError = (FlutterErrorDetails details) {
    FlutterError.presentError(details);
    if (kReleaseMode)
      exit(1);
  };
  runApp(MyApp());
}

// rest of `flutter create` code...

class MyApp extends StatelessWidget {
...
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      ...
      builder: (BuildContext context, Widget widget) {
        Widget error = Text('...rendering error...');
        if (widget is Scaffold || widget is Navigator)
          error = Scaffold(body: Center(child: error));
        ErrorWidget.builder = (FlutterErrorDetails errorDetails) => error;
        return widget;
      },
    );
  }
}

OutlinedButton(
  child: Text('Click me!'),
  onPressed: () async {
    final channel = const MethodChannel('crashy-custom-channel');
    await channel.invokeMethod('blah');
  },
),

import 'dart:async';

void main() {
  runZonedGuarded(() {
    runApp(MyApp());
  }, (Object error, StackTrace stack) {
    myBackend.sendError(error, stack);
  });
}

runZonedGuarded(() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}