dart2wasm: Null check operator used on a null value at AstCodeGenerator._setupLocalParameters

[davidmigloz]

I am trying to compile a Flutter web app using the WASM target, but I am getting the following exception:

$ flutter build web --wasm --release

Target dart2wasm failed: ProcessException: Process exited abnormally with exit
code 255:
Unhandled exception:
Null check operator used on a null value
#0      AstCodeGenerator._setupLocalParameters.setupParamLocal
(package:dart2wasm/code_generator.dart:287)
...

Full stack trace

Target dart2wasm failed: ProcessException: Process exited abnormally with exit
code 255:
Unhandled exception:
Null check operator used on a null value
#0      AstCodeGenerator._setupLocalParameters.setupParamLocal
(package:dart2wasm/code_generator.dart:287)
#1      AstCodeGenerator._setupLocalParameters
(package:dart2wasm/code_generator.dart:358)
#2      AstCodeGenerator.setupParameters
(package:dart2wasm/code_generator.dart:387)
#3      AstCodeGenerator.setupParametersAndContexts
(package:dart2wasm/code_generator.dart:411)
#4      SynchronousProcedureCodeGenerator.generateInternal
(package:dart2wasm/code_generator.dart:3234)
#5      AstCodeGenerator.generate (package:dart2wasm/code_generator.dart:212)
#6      AstCompilationTask.run (package:dart2wasm/translator.dart:1454)
#7      Translator.translate (package:dart2wasm/translator.dart:414)
#8      compileToModule (package:dart2wasm/compile.dart:253)
<asynchronous suspension>
#9      generateWasm (package:dart2wasm/generate_wasm.dart:80)
<asynchronous suspension>
#10     main
(file:///Volumes/Work/s/w/ir/x/w/sdk/pkg/dart2wasm/bin/dart2wasm.dart:10)
<asynchronous suspension>
  Command: /Users/xxx/repos/flutter/bin/cache/dart-sdk/bin/dart compile
  wasm
  --packages=/Users/xxx/xxx/xxx/app/.dart_tool/package_config.json
  --extra-compiler-option=--platform=/Users/xxx/repos/flutter/bin/cache/
  flutter_web_sdk/kernel/dart2wasm_platform.dill -Ddart.vm.profile=false
  -Ddart.vm.product=true
  --extra-compiler-option=--delete-tostring-package-uri=dart:ui
  --extra-compiler-option=--delete-tostring-package-uri=package:flutter
  --extra-compiler-option=--import-shared-memory
  --extra-compiler-option=--shared-memory-max-pages=32768
  -DFLUTTER_WEB_USE_SKIA=false -DFLUTTER_WEB_USE_SKWASM=true
  -DFLUTTER_WEB_CANVASKIT_URL=https://www.gstatic.com/flutter-canvaskit/cf56914b
  326edb0ccb123ffdc60f00060bd513fa/
  --extra-compiler-option=--depfile=/Users/xxx/xxx/xxx/app/xxx
  /.dart_tool/flutter_build/d12e46422de5e2738ca11434eca92746/dart2wasm.d -O4
  --strip-wasm --no-source-maps -o
  /Users/xxx/xxx/xxx/app/xxx/.dart_tool/flutter_build/d12e4
  6422de5e2738ca11434eca92746/main.dart.wasm
  /Users/xxx/xxx/xxx/app/xxx/.dart_tool/flutter_build/d12e4
  6422de5e2738ca11434eca92746/main.dart
#0      RunResult.throwException (package:flutter_tools/src/base/process.dart:118:5)
#1      _DefaultProcessUtils.run (package:flutter_tools/src/base/process.dart:344:19)
<asynchronous suspension>
#2      Dart2WasmTarget.build (package:flutter_tools/src/build_system/targets/web.dart:346:5)
<asynchronous suspension>
#3      _BuildInstance._invokeInternal (package:flutter_tools/src/build_system/build_system.dart:876:9)
<asynchronous suspension>
#4      Future.wait.<anonymous closure> (dart:async/future.dart:528:21)
<asynchronous suspension>
#5      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:814:32)
<asynchronous suspension>
#6      Future.wait.<anonymous closure> (dart:async/future.dart:528:21)
<asynchronous suspension>
#7      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:814:32)
<asynchronous suspension>
#8      FlutterBuildSystem.build (package:flutter_tools/src/build_system/build_system.dart:637:16)
<asynchronous suspension>
#9      WebBuilder.buildWeb (package:flutter_tools/src/web/compile.dart:93:34)
<asynchronous suspension>
#10     BuildWebCommand.runCommand (package:flutter_tools/src/commands/build_web.dart:253:5)
<asynchronous suspension>
#11     FlutterCommand.run.<anonymous closure> (package:flutter_tools/src/runner/flutter_command.dart:1558:27)
<asynchronous suspension>
#12     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#13     CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#14     FlutterCommandRunner.runCommand.<anonymous closure> (package:flutter_tools/src/runner/flutter_command_runner.dart:496:9)
<asynchronous suspension>
#15     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#16     FlutterCommandRunner.runCommand (package:flutter_tools/src/runner/flutter_command_runner.dart:431:5)
<asynchronous suspension>
#17     run.<anonymous closure>.<anonymous closure> (package:flutter_tools/runner.dart:98:11)
<asynchronous suspension>
#18     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#19     main (package:flutter_tools/executable.dart:99:3)
<asynchronous suspension>

Compiling lib/main.dart for the Web...                             50.6s
Error: Failed to compile application for the Web.

Dart Info

#### General info

- Dart 3.7.2 (stable) (Tue Mar 11 04:27:50 2025 -0700) on "macos_arm64"
- on macos / Version 15.4.1 (Build 24E263)
- locale is en-US

#### Project info

- sdk constraint: '3.7.2'

#### Process info

|  Memory |  CPU | Elapsed time | Command line                                                                               |
| ------: | ---: | -----------: | ------------------------------------------------------------------------------------------ |
|   26 MB | 0.0% |     01:21:24 | dart devtools --machine --dtd-uri=ws:<path>/umfSw9s6aN0=                                   |
| 1528 MB | 0.0% |     01:21:24 | dart language-server --client-id=IntelliJ-IDEA --client-version=IU-243.26053.27 --protocol=analyzer |
|   29 MB | 0.0% |     01:21:24 | dart tooling-daemon --machine                                                              |
|   87 MB | 0.0% |     01:21:06 | flutter_tools.snapshot daemon                                                              |

Unfortunately, it is not an open source app and I haven't managed to create a minimal reproducible example. Please let me know if you need any additional details.

[srujzs]

I believe the corresponding code at that Dart version is here:

sdk/pkg/dart2wasm/lib/code_generator.dart

Line 287 in d80fab4

translateExpression(variable.initializer!, local.type);

, and that assertion still exists in the latest version so it's possible this is still reproducible.

[osa1]

It's difficult to debug this without a repro, but I suspect the issue is here, when we specify which parameters to use for type checking:

sdk/pkg/dart2wasm/lib/translator.dart

Lines 1298 to 1307 in 669bcef

	Procedure? forwardingTarget;
	if (member is Procedure && member.isForwardingStub) {
	forwardingTarget = member.concreteForwardingStubTarget as Procedure?;
	}
	final List<TypeParameter> typeParametersToTypeCheck =
	forwardingTarget?.typeParameters ?? typeParameters;
	final List<VariableDeclaration> positionalToTypeCheck =
	forwardingTarget?.function.positionalParameters ?? positional;
	final List<VariableDeclaration> namedToTypeCheck =
	forwardingTarget?.function.namedParameters ?? named;

I don't understand what this is doing exactly as concreteForwardingStubTarget is undocumented:

sdk/pkg/kernel/lib/src/ast/members.dart

Lines 1170 to 1173 in 669bcef

	Member? get concreteForwardingStubTarget =>
	stubKind == ProcedureStubKind.ConcreteForwardingStub
	? stubTargetReference?.asMember
	: null;

But from the crash, it looks like when we return different VariableDeclarations than the parameters in the first code above, we return VariableDeclarations without initializers for optional parameters.

I think the fix might be using the original VariableDeclarations (from the Member) rather than the ones returned by getParametersToCheck for setting up locals.

@mkustermann might already know what the issue is as he wrote the first code above.

[mkustermann]

My gut feeling is that concreteForwardingStubTarget isn't at fault here: Whether we take it from the member or the forwarding target member. In both cases optional parameters should have a default value (even if it's implicit default value of null) - I believe the CFE ensures all optionals get default values in the initializer.

It could be that some kernel transformer somewhere doesn't maintain the invariant and creates optional variables without initializers.

@davidmigloz Is there any chance you could provide us with a minimal reproduction of this issue?

[davidmigloz]

I've just tried again with Flutter 3.32.0 / Dart 3.8.0, but the issue is still happening:

Full stack trace

Target dart2wasm failed: ProcessException: Process exited abnormally with exit code 255:
Unhandled exception:
Null check operator used on a null value
#0      AstCodeGenerator._setupLocalParameters.setupParamLocal (package:dart2wasm/code_generator.dart:283)
#1      AstCodeGenerator._setupLocalParameters (package:dart2wasm/code_generator.dart:352)
#2      AstCodeGenerator.setupParameters (package:dart2wasm/code_generator.dart:382)
#3      AstCodeGenerator.setupParametersForNormalEntry (package:dart2wasm/code_generator.dart:388)
#4      SynchronousProcedureCodeGenerator._makeNonMultiEntryPointFunction (package:dart2wasm/code_generator.dart:3402)
#5      SynchronousProcedureCodeGenerator.generateInternal (package:dart2wasm/code_generator.dart:3309)
#6      AstCodeGenerator.generate (package:dart2wasm/code_generator.dart:217)
#7      AstCompilationTask.run (package:dart2wasm/translator.dart:1850)
#8      Translator.drainCompletionQueue (package:dart2wasm/translator.dart:483)
#9      Translator.translate (package:dart2wasm/translator.dart:508)
#10     compileToModule (package:dart2wasm/compile.dart:317)
<asynchronous suspension>
#11     generateWasm (package:dart2wasm/generate_wasm.dart:80)
<asynchronous suspension>
#12     main (file:///Volumes/Work/s/w/ir/x/w/sdk/pkg/dart2wasm/bin/dart2wasm.dart:10)
<asynchronous suspension>
  Command: /Users/xxx/repos/flutter/bin/cache/dart-sdk/bin/dart compile wasm --packages=/Users/xxx/xxx/xxx/app/.dart_tool/package_config.json
  --extra-compiler-option=--platform=/Users/xxx/repos/flutter/bin/cache/flutter_web_sdk/kernel/dart2wasm_platform.dill -Ddart.vm.profile=false
  -Ddart.vm.product=true --extra-compiler-option=--delete-tostring-package-uri=dart:ui --extra-compiler-option=--delete-tostring-package-uri=package:flutter
  --extra-compiler-option=--import-shared-memory --extra-compiler-option=--shared-memory-max-pages=32768 -DFLUTTER_VERSION=3.32.0 -DFLUTTER_CHANNEL=stable
  -DFLUTTER_GIT_URL=https://github.com/flutter/flutter.git -DFLUTTER_FRAMEWORK_REVISION=be698c48a6 -DFLUTTER_ENGINE_REVISION=1881800949 -DFLUTTER_DART_VERSION=3.8.0
  -DFLUTTER_WEB_USE_SKIA=false -DFLUTTER_WEB_USE_SKWASM=true
  -DFLUTTER_WEB_CANVASKIT_URL=https://www.gstatic.com/flutter-canvaskit/18818009497c581ede5d8a3b8b833b81d00cebb7/
  --extra-compiler-option=--depfile=/Users/xxx/xxx/xxx/app/xxx_admin_app/.dart_tool/flutter_build/0f140d15ddbd21b8d434755c28533a29/dart2wasm.d -O2 --strip-wasm
  --no-source-maps -o /Users/xxx/xxx/xxx/app/xxx_admin_app/.dart_tool/flutter_build/0f140d15ddbd21b8d434755c28533a29/main.dart.wasm
  /Users/xxx/xxx/xxx/app/xxx_admin_app/.dart_tool/flutter_build/0f140d15ddbd21b8d434755c28533a29/main.dart
#0      RunResult.throwException (package:flutter_tools/src/base/process.dart:118:5)
#1      _DefaultProcessUtils.run (package:flutter_tools/src/base/process.dart:344:19)
<asynchronous suspension>
#2      Dart2WasmTarget.build (package:flutter_tools/src/build_system/targets/web.dart:352:5)
<asynchronous suspension>
#3      _BuildInstance._invokeInternal (package:flutter_tools/src/build_system/build_system.dart:873:9)
<asynchronous suspension>
#4      Future.wait.<anonymous closure> (dart:async/future.dart:525:21)
<asynchronous suspension>
#5      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:811:32)
<asynchronous suspension>
#6      Future.wait.<anonymous closure> (dart:async/future.dart:525:21)
<asynchronous suspension>
#7      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:811:32)
<asynchronous suspension>
#8      FlutterBuildSystem.build (package:flutter_tools/src/build_system/build_system.dart:629:16)
<asynchronous suspension>
#9      WebBuilder.buildWeb (package:flutter_tools/src/web/compile.dart:91:34)
<asynchronous suspension>
#10     BuildWebCommand.runCommand (package:flutter_tools/src/commands/build_web.dart:246:5)
<asynchronous suspension>
#11     FlutterCommand.run.<anonymous closure> (package:flutter_tools/src/runner/flutter_command.dart:1551:27)
<asynchronous suspension>
#12     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#13     CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#14     FlutterCommandRunner.runCommand.<anonymous closure> (package:flutter_tools/src/runner/flutter_command_runner.dart:494:9)
<asynchronous suspension>
#15     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#16     FlutterCommandRunner.runCommand (package:flutter_tools/src/runner/flutter_command_runner.dart:431:5)
<asynchronous suspension>
#17     run.<anonymous closure>.<anonymous closure> (package:flutter_tools/runner.dart:98:11)
<asynchronous suspension>
#18     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#19     main (package:flutter_tools/executable.dart:102:3)
<asynchronous suspension>

Compiling lib/main.dart for the Web...                             34.7s
Error: Failed to compile application for the Web.

---

@davidmigloz Is there any chance you could provide us with a minimal reproduction of this issue?

@mkustermann I'd like to provide a minimal reproduction. However, given the size of the project, it's tough to know where to start to effectively isolate the issue.

I've tried running it with --verbose, but unfortunately, that didn't provide additional useful details. Is there another method I could use to pinpoint the exact file/line causing the exception? That would be a great help in creating the reproduction.