[BUG] Attempt to read from field 'java.lang.String io.flutter.embedding.engine.loader.FlutterApplicationInfo.flutterAssetsDir' on a null object reference

[KorbinianMossandl]

Your Environment

• Plugin version: 0.6.0
• Platform: Android
• OS version: 10.0
• Device manufacturer / model: any
• Flutter info (flutter info, flutter doctor):

[✓] Flutter (Channel master, 1.23.0-19.0.pre.95, on Linux, locale en_US.UTF-8)
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Android Studio (version 4.0)
[✓] Connected device (1 available)

• Plugin config

BackgroundFetchConfig(
          minimumFetchInterval: 30,
          stopOnTerminate: false,
          enableHeadless: true,
          startOnBoot: true,
          requiredNetworkType: NetworkType.ANY,
        )

To Reproduce
Steps to reproduce the behavior:

1. register headless task with flutter version >=1.22.0
2. close app
3. trigger headless task

Debug logs

2020-10-19 14:26:10.786 6164-6164/com.test.app.demo E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.test.app.demo, PID: 6164
    java.lang.NullPointerException: Attempt to read from field 'java.lang.String io.flutter.embedding.engine.loader.FlutterApplicationInfo.flutterAssetsDir' on a null object reference
        at io.flutter.embedding.engine.loader.FlutterLoader.findAppBundlePath(FlutterLoader.java:324)
        at io.flutter.view.FlutterMain.findAppBundlePath(FlutterMain.java:98)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.startBackgroundIsolate(HeadlessTask.java:128)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.dispatch(HeadlessTask.java:101)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.run(HeadlessTask.java:95)
        at android.os.Handler.handleCallback(Handler.java:907)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:216)
        at android.app.ActivityThread.main(ActivityThread.java:7506)
        at java.lang.reflect.Method.invoke(Native Method)
    	at com.android.internal.os.RuntimeInit$MethodAndArgsCal

Additional context
At first glance when you search via google this seems to be a bug in flutter itself
flutter/flutter#64458
but the fix is already on master and as you can see, i still run into this issue.
everything works fine as long as the app is still running and a backround task is triggered. only the headless one crashes with above error.

[kanczler]

I'm having the same issue. On my own device (Huawei P30 Pro), there are no problems, headless tasks run just fine, but on an android 7.1.1 emulator and several other devices whenever the headless task is triggered, the application crashes with the exact same exception.

[christocracy]

@kanczler flutter doctor

[kanczler]

My apologies!

flutter doctor:

[✓] Flutter (Channel stable, 1.22.2, on Mac OS X 10.15.7 19H2, locale en-GB)  
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)  
[✓] Xcode - develop for iOS and macOS (Xcode 12.1)  
[✓] Android Studio (version 4.1)  
[✓] IntelliJ IDEA Ultimate Edition (version 2020.2.3)  
[✓] VS Code (version 1.50.0)  
[✓] Connected device (1 available)  

[christocracy]

I'm having the same issue

Post the exact stacktrace

[kanczler]

Stack trace:

2020-10-21 15:49:48.261 32397-32397/hu.bravobased E/AndroidRuntime: FATAL EXCEPTION: main
    Process: hu.bravobased, PID: 32397
    java.lang.NullPointerException: Attempt to read from field 'java.lang.String io.flutter.embedding.engine.loader.FlutterApplicationInfo.flutterAssetsDir' on a null object reference
        at io.flutter.embedding.engine.loader.FlutterLoader.findAppBundlePath(FlutterLoader.java:324)
        at io.flutter.view.FlutterMain.findAppBundlePath(FlutterMain.java:97)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.startBackgroundIsolate(HeadlessTask.java:128)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.dispatch(HeadlessTask.java:101)
        at com.transistorsoft.flutter.backgroundfetch.HeadlessTask.run(HeadlessTask.java:95)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

[christocracy]

Boot the /example in this repo.

[christocracy]

I have just used flutter create foo to create a fresh new Hello World app. with flutter doctor:

[✓] Flutter (Channel stable, 1.22.2, on Mac OS X 10.15.6 19G2021, locale en-CA)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Xcode - develop for iOS and macOS (Xcode 12.0.1)
[✓] Android Studio (version 4.0)
[✓] VS Code (version 1.45.1)
[✓] Connected device (3 available)

I cannot reproduce this. How old is your app? You might have old code in your app from a previously generated flutter app that hasn't been properly upgraded.

[KorbinianMossandl]

yes, our app is about 1 year old, be we regularly maintain it, especially updates to flutter and the project structure.
the problem has only recently started with our latest release which made a switch from flutter 1.12 to 1.22.0.
Also as @kanczler mentioned it does not happen on every device.
Any ideas what this could be causing and why it only happens with flutter 1.22.0+ are greatly appreciated.
Will play around with the example project in the mean time and see if i can repdroduce.

[christocracy]

If your app was generated quite a long time ago, I suggest using flutter create to generate a fresh new app and migrate your /lib code into the new app and migrate your dependencies into the fresh pubspec.yaml.

[KorbinianMossandl]

not an option. we made so many customizations to the project structure for flavors and such. it would take forever to get everything to this point again.

[christocracy]

Then I suggest you flutter create foo and observe the difference between key android files, such as:

• android/build.gradle
• android/settings.gradle
• android/app/build.gradle

You will want to migrate changes into your app.

Are you using a custom Application.java / Application.kt? If so, post it.

[christocracy]

we made so many customizations to the project structure for flavors and such

This is not hard to migrate.

[KorbinianMossandl]

Then I suggest you flutter create foo and observe the difference between key android files, such as:

• android/build.gradle
• android/settings.gradle
• android/app/build.gradle

You will want to migrate changes into your app.

Are you using a custom Application.java / Application.kt? If so, post it.

we don't use a custom application.kt
but i will try to spot any differences between a fresh project and ours. that is the best lead so far.
will update with the results.

[christocracy]

I made a mistake in the files to check. Corrected above (ie: android/app/build.gradle)

[KorbinianMossandl]

ok, i looked at all the files, made changes so everything is exactly like in a newly created project, but the issue still exists.

but what i did is take our current project and move back to flutter 1.20.0 and the issue is completely gone.
so something in flutter 1.22.x broke/changed something that is causing the issues with background_fetch headless tasks.

it would be greatly appreciated if you could take another look.