(With pull request) reaction<Sth?>() errors: type 'Null' is not a subtype of type 'T' of 'value' reaction=Instance of 'ReactionImpl'

[fzyzcjy]

Hi thanks for the lib! We have error when executing the following minimum reproducible sample:

void oops() {
  // mainContext.config = ReactiveConfig(disableErrorBoundaries: true);

  mainContext.onReactionError((e, reaction) {
    print('onReactionError error.type=${e.runtimeType} error=$e reaction=$reaction');
    if (e is MobXCaughtException) {
      debugPrint(
          'e.exception.runtimeType=${e.exception.runtimeType} e.exception=${e.exception} e.stackTrace=${e.stackTrace}');
    }
  });

  print('hi 0');
  reaction<int?>(
    (Reaction r) {
      print('hi 1');
      return null;
      // return 42; // if this then ok
    },
    (int? val) {
      print('hi 2');
    },
    fireImmediately: true,
  );
  print('hi 3');
}

The full code:

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

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  void initState() {
    super.initState();
    oops();
  }

  Widget build(BuildContext context) {
    return Container();
  }
}

// and copy `oops()` here

The error:

Performing hot restart...
Syncing files to device iPhone 11...
Restarted application in 438ms.
flutter: hi 0
flutter: hi 1
flutter: onReactionError error.type=MobXCaughtException error=MobXCaughtException: type 'Null' is not a subtype of type 'T' of 'value' reaction=Instance of 'ReactionImpl'
flutter: e.exception.runtimeType=_TypeError e.exception=type 'Null' is not a subtype of type 'T' of 'value' e.stackTrace=#0      _TypeError._throwNew (dart:core-patch/errors_patch.dart:98:34)
flutter: #1      Function._apply (dart:core-patch/function_patch.dart:11:30)
flutter: #2      Function.apply (dart:core-patch/function_patch.dart:34:12)
flutter: #3      Action.call (package:mobx/src/core/action.dart:53:25)
flutter: #4      createReaction.reactionRunner (package:mobx/src/core/reaction_helper.dart:108:19)
flutter: #5      createReaction.<anonymous closure> (package:mobx/src/core/reaction_helper.dart:120:21)
flutter: #6      ReactionImpl._run (package:mobx/src/core/reaction.dart:117:22)
flutter: #7      ReactiveContext._runReactionsInternal (package:mobx/src/core/context.dart:345:18)
flutter: #8      ReactiveContext.runReactions (package:mobx/src/core/context.dart:319:5)
flutter: #9      ReactionImpl.schedule (package:mobx/src/core/reaction.dart:157:9)
flutter: #10     createReaction (package:mobx/src/core/reaction_helper.dart:139:7)
flutter: #11     reaction (package:mobx/src/api/reaction.dart:54:5)
flutter: #12     oops (package:bug/main.dart:35:3)
flutter: #13     _MyAppState.initState (package:bug/main.dart:15:5)
flutter: #14     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4632:57)
flutter: #15     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4469:5)
flutter: #16     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3541:14)
flutter: #17     Element.updateChild (package:flutter/src/widgets/framework.dart:3306:18)
flutter: #18     RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1182:16)
flutter: #19     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1153:5)
flutter: #20     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1095:18)
flutter: #21     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2647:19)
flutter: #22     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1094:13)
flutter: #23     WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:934:7)
flutter: #24     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:915:7)
flutter: #25     _rootRun (dart:async/zone.dart:1346:47)
flutter: #26     _CustomZone.run (dart:async/zone.dart:1258:19)
flutter: #27     _CustomZone.runGuarded (dart:async/zone.dart:1162:7)
flutter: #28     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1202:23)
flutter: #29     _rootRun (dart:async/zone.dart:1354:13)
flutter: #30     _CustomZone.run (dart:async/zone.dart:1258:19)
flutter: #31     _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:1186:23)
flutter: #32     Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18:15)
flutter: #33     _Timer._runTimers (dart:isolate-patch/timer_impl.dart:395:19)
flutter: #34     _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:426:5)
flutter: #35     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
flutter:
flutter: hi 3