Unsupported operation: Undetermined Nullability

[hamishjohnson]

Describe the bug
This code crashes when you build for web, but not for mobile.

I'm able to reproduce with the Riverpod package, but I don't know enough about the problem to reproduce without riverpod. I opened an issue there but maintainer Remi Rousselet believes it's a dart issue, so I'm reposting here.

To Reproduce

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

void main() {
  runApp(const ProviderScope(child: MyApp()));
}

class MyApp extends ConsumerWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final value = ref.watch(stringProvider);

    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(body: Text(value.value ?? '')),
    );
  }
}

const condition = false;

final stringProvider = FutureProvider<String>((ref) {
  if (condition) return 'child'; // <==== This line is what causes the bug

  return future();
});

Future<String> future() async {
  await Future.delayed(const Duration(seconds: 1));

  return 'done';
}

Expected behavior
Run on mobile, works fine. Run on web, compile crashes.

When adding 'async' to the provided create function, it works fine.

Error

Launching lib[/main.dart]() on Chrome in debug mode...
lib/main.dart:1
Unhandled exception:
Unsupported operation: Undetermined Nullability
#0      ProgramCompiler._emitFutureOrType (package:dev_compiler[/src/kernel/compiler.dart:3057:7]())
#1      ProgramCompiler.visitFutureOrType (package:dev_compiler[/src/kernel/compiler.dart:2948:11]())
#2      FutureOrType.accept (package:kernel[/ast.dart:11519:42]())
#3      ProgramCompiler._emitType (package:dev_compiler[/src/kernel/compiler.dart:2847:54]())
#4      ProgramCompiler.visitFunctionType (package:dev_compiler[/src/kernel/compiler.dart:3138:22]())
#5      ProgramCompiler._emitFunctionTagged (package:dev_compiler[/src/kernel/compiler.dart:2803:19]())
#6      ProgramCompiler.visitFunctionExpression (package:dev_compiler[/src/kernel/compiler.dart:6186:12]())
#7      FunctionExpression.accept (package:kernel[/ast.dart:8292:44]())
#8      ProgramCompiler._visitExpression (package:dev_compiler[/src/kernel/compiler.dart:3870:20]())
#9      ProgramCompiler._emitArgumentList (package:dev_compiler[/src/kernel/compiler.dart:5614:11]())

#10     ProgramCompiler.visitConstructorInvocation (package:dev_compiler[/src/kernel/compiler.dart:5761:9]())
#11     ConstructorInvocation.accept (package:kernel[/ast.dart:6392:44]())
#12     ProgramCompiler._visitExpression (package:dev_compiler[/src/kernel/compiler.dart:3870:20]())
#13     ProgramCompiler._visitInitializer (package:dev_compiler[/src/kernel/compiler.dart:1786:11]())
#14     ProgramCompiler._emitStaticFieldInitializer.<anonymous closure> (package:dev_compiler[/src/kernel/compiler.dart:2366:23]())
#15     ProgramCompiler._withLetScope (package:dev_compiler[/src/kernel/compiler.dart:2376:25]())
#16     ProgramCompiler._emitStaticFieldInitializer (package:dev_compiler[/src/kernel/compiler.dart:2364:40]())
#17     ProgramCompiler._emitLazyFields (package:dev_compiler[/src/kernel/compiler.dart:2339:43]())
#18     ProgramCompiler._emitTopLevelFields (package:dev_compiler[/src/kernel/compiler.dart:2323:21]())
#19     ProgramCompiler._emitLibrary (package:dev_compiler[/src/kernel/compiler.dart:573:7]())
#20     List.forEach (dart:core-patch[/growable_array.dart:433:8]())
#21     ProgramCompiler.emitModule (package:dev_compiler[/src/kernel/compiler.dart:431:15]())
#22     JavaScriptBundler.compile (package:frontend_server[/src/javascript_bundle.dart:152:33]())
<asynchronous suspension>
#23     FrontendCompiler.writeJavascriptBundle (package:frontend_server[/frontend_server.dart:665:32]())

<asynchronous suspension>
#24     FrontendCompiler.compile (package:frontend_server[/frontend_server.dart:573:9]())
<asynchronous suspension>
#25     listenAndCompile.<anonymous closure> (package:frontend_server[/frontend_server.dart:1154:11]())
<asynchronous suspension>

the Dart compiler exited unexpectedly.
Exited (sigterm)
Failed to compile application.

Edit:

I forgot to add that I was able to fix the issue in the meantime by wrapping the value in a future, like so

Replace
if (condition) return 'child'; // <==== This line is what causes the bug

With
if (condition) return Future.value('child');

In case that helps anybody

[caleb654]

Also using Riverpod and getting this issue. I am able to build a release build, but in debug, it crashes every time.

[caleb654]

@hamishjohnson How did you figure out which line was causing the issue? I'm not able to get any details about that in the stack trace?

[hamishjohnson]

I was just trial and errored it. By the way, I updated the post with a fix (wrapping in a Future) in case that helps you. Check if you have any FutureProviders that aren't returning a Future. If not, it might be an unrelated issue

But the bug still exists

[devoncarew]

Highlighting the stack trace from above:

Launching lib[/main.dart]() on Chrome in debug mode...
lib/main.dart:1
Unhandled exception:
Unsupported operation: Undetermined Nullability
#0      ProgramCompiler._emitFutureOrType (package:dev_compiler[/src/kernel/compiler.dart:3057:7]())
#1      ProgramCompiler.visitFutureOrType (package:dev_compiler[/src/kernel/compiler.dart:2948:11]())
#2      FutureOrType.accept (package:kernel[/ast.dart:11519:42]())
#3      ProgramCompiler._emitType (package:dev_compiler[/src/kernel/compiler.dart:2847:54]())
#4      ProgramCompiler.visitFunctionType (package:dev_compiler[/src/kernel/compiler.dart:3138:22]())
#5      ProgramCompiler._emitFunctionTagged (package:dev_compiler[/src/kernel/compiler.dart:2803:19]())
#6      ProgramCompiler.visitFunctionExpression (package:dev_compiler[/src/kernel/compiler.dart:6186:12]())
#7      FunctionExpression.accept (package:kernel[/ast.dart:8292:44]())
#8      ProgramCompiler._visitExpression (package:dev_compiler[/src/kernel/compiler.dart:3870:20]())
#9      ProgramCompiler._emitArgumentList (package:dev_compiler[/src/kernel/compiler.dart:5614:11]())

#10     ProgramCompiler.visitConstructorInvocation (package:dev_compiler[/src/kernel/compiler.dart:5761:9]())
#11     ConstructorInvocation.accept (package:kernel[/ast.dart:6392:44]())
#12     ProgramCompiler._visitExpression (package:dev_compiler[/src/kernel/compiler.dart:3870:20]())
#13     ProgramCompiler._visitInitializer (package:dev_compiler[/src/kernel/compiler.dart:1786:11]())
#14     ProgramCompiler._emitStaticFieldInitializer.<anonymous closure> (package:dev_compiler[/src/kernel/compiler.dart:2366:23]())
#15     ProgramCompiler._withLetScope (package:dev_compiler[/src/kernel/compiler.dart:2376:25]())
#16     ProgramCompiler._emitStaticFieldInitializer (package:dev_compiler[/src/kernel/compiler.dart:2364:40]())
#17     ProgramCompiler._emitLazyFields (package:dev_compiler[/src/kernel/compiler.dart:2339:43]())
#18     ProgramCompiler._emitTopLevelFields (package:dev_compiler[/src/kernel/compiler.dart:2323:21]())
#19     ProgramCompiler._emitLibrary (package:dev_compiler[/src/kernel/compiler.dart:573:7]())
#20     List.forEach (dart:core-patch[/growable_array.dart:433:8]())
#21     ProgramCompiler.emitModule (package:dev_compiler[/src/kernel/compiler.dart:431:15]())
#22     JavaScriptBundler.compile (package:frontend_server[/src/javascript_bundle.dart:152:33]())
<asynchronous suspension>
#23     FrontendCompiler.writeJavascriptBundle (package:frontend_server[/frontend_server.dart:665:32]())

<asynchronous suspension>
#24     FrontendCompiler.compile (package:frontend_server[/frontend_server.dart:573:9]())
<asynchronous suspension>
#25     listenAndCompile.<anonymous closure> (package:frontend_server[/frontend_server.dart:1154:11]())
<asynchronous suspension>

the Dart compiler exited unexpectedly.
Exited (sigterm)
Failed to compile application.

[gmpassos]

Maybe related to:
#48631

FYI: @chloestefantsova

[nshahan]

@chloestefantsova recently fixed two similar issues. One has already rolled into flutter main, but the second hasn't yet. The second fix was for an issue reported by another user with a RiverPod dependency #48631 (comment). I suspect this will be fixed by one of those changes and I'd like to verify if the issue is still present at flutter master after the second fix has rolled in.

[chloestefantsova]

I think so too, that my recent fix should resolve this issue. I try to locally verify that.

[chloestefantsova]

Ok, @whesse checked the fix that's already in the Dart SDK, and it seems to work.