Provider: markNeedsRebuild() called during build

main.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MultiProvider(
        providers: [
          ChangeNotifierProvider<ANotifier>(create: (context) => ANotifier()),
          ChangeNotifierProxyProvider<ANotifier, BNotifier>(
            create: (_) => BNotifier(),
            update: (_, aNot, bNot) {
              bNot.onANotifierUpdated(aNot);
              return bNot;
            },
          )
        ],
        child: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final bNotifier = Provider.of<BNotifier>(context);
    print("BUILDING: ${DateTime.now()}");

    return Scaffold(
      appBar: AppBar(
        title: Text(bNotifier.myValue),
      ),
      body: Center(
        child: FlatButton(
          onPressed: () async {
            final bNotifier = Provider.of<BNotifier>(context, listen: false);
            final aNotifier = Provider.of<ANotifier>(context, listen: false);

            await showModalBottomSheet<void>(
              context: context,
              builder: (context) {
                // Inject the providers to use in the bottom sheet route
                return MultiProvider(
                  providers: [
                    ChangeNotifierProvider<ANotifier>.value(value: aNotifier),
                    ChangeNotifierProvider<BNotifier>.value(value: bNotifier)
                  ],
                  builder: (context, _) {
                    return MyBottomSheet();
                  },
                );
              },
            );
          },
          child: Text("Open bottom sheet"),
          color: Colors.blue,
        ),
      ),
    );
  }
}

class MyBottomSheet extends StatelessWidget {
  const MyBottomSheet({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final bNotifier = Provider.of<BNotifier>(context);

    return Column(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        Text(bNotifier.myValue),
        FlatButton(
          onPressed: () {
            final aNotifier = Provider.of<ANotifier>(context, listen: false);
            aNotifier.update();
          },
          child: Text("Trigger error"),
          color: Colors.blue,
        ),
      ],
    );
  }
}

class ANotifier extends ChangeNotifier {
  void update() {
    print("UPDATE A");
    notifyListeners();
  }
}

class BNotifier extends ChangeNotifier {
  int _value = 0;

  BNotifier();

  void update() {
    _value++;
    notifyListeners();
  }

  String get myValue => "BNotifier State = $_value";

  void onANotifierUpdated(ANotifier a) {
    print("onANotifierUpdated...");
    notifyListeners();
  }
}