fix: using flutter plugins fail when accessing assetManager

[nietsmmar]

Description

I try to embed my flutter app but get this error:

Uncaught DartError: Unexpected null value.
    at Object.throw_ [as throw] (errors.dart:274:3)
    at Object.nullCheck (profile.dart:118:39)
    at get engineAssetManager (initialization.dart:192:82)
    at get assetManager (asset_manager.dart:9:34)
    at new package_info_plus_web.PackageInfoPlusWebPlugin.new (package_info_plus_web.dart:20:45)
    at PackageInfoPlusWebPlugin.registerWith (package_info_plus_web.dart:24:36)
    at main.client.options.dart:58:53
    at Jaspr.initializeApp (options.dart:13:25)
    at Object.main (main.client.dart:13:9)
    at main (main.client.dart:11:16)
    at main.client.dart.bootstrap.js:1188:10
    at Array.forEach (<anonymous>)
    at window.$dartRunMain (main.client.dart.bootstrap.js:1187:32)
    at <anonymous>:1:8
    at Object.runMain (client.js:9953:10)
    at client.js:26774:19
    at _wrapJsFunctionForAsync_closure.$protected (client.js:3956:15)
    at _wrapJsFunctionForAsync_closure.call$2 (client.js:12880:12)
    at Object._asyncStartSync (client.js:3920:20)
    at main__closure6.$call$body$main__closure (client.js:26783:16)
    at main__closure6.call$1 (client.js:26701:19)
    at StaticClosure._rootRunUnary (client.js:4379:16)
    at _CustomZone.runUnary$2$2 (client.js:14383:39)
    at _CustomZone.runUnaryGuarded$1$2 (client.js:14330:14)
    at _ControllerSubscription._sendData$1 (client.js:13865:19)
    at _DelayedData.perform$1 (client.js:14046:59)
    at _PendingEvents_schedule_closure.call$0 (client.js:14105:14)
    at Object._microtaskLoop (client.js:4211:24)
    at StaticClosure._startMicrotaskLoop (client.js:4217:11)
    at _AsyncRun__initializeScheduleImmediate_internalCallback.call$1 (client.js:12758:9)
throw_ @ errors.dart:274
nullCheck @ profile.dart:118
get engineAssetManager @ initialization.dart:192
get assetManager @ asset_manager.dart:9
package_info_plus_web.PackageInfoPlusWebPlugin.new @ package_info_plus_web.dart:20
registerWith @ package_info_plus_web.dart:24
(anonymous) @ main.client.options.dart:58
initializeApp @ options.dart:13
main @ main.client.dart:13
main @ main.client.dart:11
(anonymous) @ main.client.dart.bootstrap.js:1188
window.$dartRunMain @ main.client.dart.bootstrap.js:1187
(anonymous) @ VM1720:1
runMain @ client.js:9953
(anonymous) @ client.js:26774
(anonymous) @ client.js:3956
call$2 @ client.js:12880
_asyncStartSync @ client.js:3920
$call$body$main__closure @ client.js:26783
call$1 @ client.js:26701
_rootRunUnary @ client.js:4379
runUnary$2$2 @ client.js:14383
runUnaryGuarded$1$2 @ client.js:14330
_sendData$1 @ client.js:13865
perform$1 @ client.js:14046
call$0 @ client.js:14105
_microtaskLoop @ client.js:4211
_startMicrotaskLoop @ client.js:4217
call$1 @ client.js:12758
childList
call$1 @ client.js:12768
_scheduleAsyncCallback @ client.js:4231
_rootScheduleMicrotask @ client.js:4426
scheduleMicrotask @ client.js:4260
schedule$1 @ client.js:14077
_addPending$1 @ client.js:13856
_sendData$1 @ client.js:13666
_add$1 @ client.js:13569
add$1 @ client.js:13526
_onIncomingMessage$1 @ client.js:25582
(anonymous) @ client.js:1598
_callDartFunctionFast1 @ client.js:7673
(anonymous) @ client.js:7649Understand this error

I tracked it down to this dependency: matomo_tracker: ^6.1.0

Steps To Reproduce

1. Checkout this minimal reproducible example: https://github.com/nietsmmar/jaspr_test
2. run jaspr serve
3. click on the test page in the tab bar
4. check browser dev-console and see the error
5. Flutter app is never shown

Doctor Output

[✓] Jaspr CLI (Version 0.22.0)
  • Dart Version 3.10.4 (stable) (Tue Dec 9 00:01:55 2025 -0800) on "linux_x64" at /home/xunil/dev/flutter/bin/cache/dart-sdk/bin/dart
  • Running on linux Linux 6.12.59 #1-NixOS SMP PREEMPT_DYNAMIC Mon Nov 24 09:36:08 UTC 2025 - Locale en_US.UTF-8
  • Analytics: Enabled
[✓] Current Project
  • Dependencies on core packages:
    • jaspr: ^0.22.0
    • jaspr_builder: ^0.22.0 (dev)
    • jaspr_flutter_embed: ^0.4.10
    • jaspr_router: ^0.8.1
  • Rendering mode: static
  • Flutter mode: embedded

Expected Behavior

Flutter app is shown

[schultek]

I found the cause of this. Flutters assetManager isn't initialized when using flutter plugins with Jaspr, since this only happens during engine startup.

I don't have a fix for this rn as this is deep in the engine, finding a solution will probably take a while.