Fatal crash - NullPointerException: Attempt to read from field 'androidx.recyclerview.widget.RecyclerView be.digitalia.fosdem.fragments.RecyclerViewFragment$ViewHolder.recyclerView' on a null object reference

[tingsu]

Hi,

Recently, I tested fosdem (v1.6.2) and encountered a fatal crash multuiple times. I tried to reproduce this issue with the action traces dumped by my testing tool, but I cannot reproduce it deterministically (also on the latest version v2.0.1). I guess the root cause may be related to the timing of actions. I posted the issue here. Let me know if you need more info. Thanks.

Event traces

GUI page where the crash happens (this crash happens after the last action in the above video was executed)

Stack trace:

 FATAL EXCEPTION: main
 Process: be.digitalia.fosdem, PID: 12643
 java.lang.NullPointerException: Attempt to read from field 'androidx.recyclerview.widget.RecyclerView be.digitalia.fosdem.fragments.RecyclerViewFragment$ViewHolder.recyclerView' on a null object reference
    at be.digitalia.fosdem.fragments.RecyclerViewFragment.getRecyclerView(RecyclerViewFragment.java:129)
    at be.digitalia.fosdem.fragments.BaseLiveListFragment$1.run(BaseLiveListFragment.java:54)
    at androidx.paging.AsyncPagedListDiffer.onCurrentListChanged(AsyncPagedListDiffer.java:403)
    at androidx.paging.AsyncPagedListDiffer.latchPagedList(AsyncPagedListDiffer.java:392)
    at androidx.paging.AsyncPagedListDiffer$2$1.run(AsyncPagedListDiffer.java:345)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

[cbeyls]

Thank you for your report!

According to the stack trace you provided, the problem was caused by a null ViewHolder in BaseLiveListFragment or RecyclerViewFragment without a proper null check. As far as I can tell, this may not happen in version 2.0.1 since the Kotlin code properly checks for null in both classes.

I also recently refactored the way ViewHolders and Views are accessed in fragments to avoid this kind of problems so I believe the bug has already been fixed.

Can you test the version on the master branch to confirm there is no more bug, and open a new issue with the correct stack trace if you still manage to reproduce it? Thank you.

[cbeyls]

Note: It looks like the null check is also present in version 1.7.3. So only version 1.6.2 could crash with that stack trace.

[tingsu]

Thanks for your confirmation and reply, @cbeyls ! Yes, I noted you very recently refactored this part. I will try to test on the master version again to see whether this issue still exists. Thanks.