AppStartTrace.onActivityResumed : java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewTreeObserver android.view.View.getViewTreeObserver()' on a null object reference

[REQUIRED] Step 2: Describe your environment

Firebase Component: Performance
Component version: 20.5.1
Other Firebase components used: Analytics, Crashlytics, DynamicLinks, Messaging
(several other third party plugins for ads, authentication, customer support)
Unity 2022.3.27f1 but happened with previous versions as well.

[REQUIRED] Step 3: Describe the problem

In our live app built with Unity (2022.3.27f1), our top crash (600 events daily but less than 1% of our sessions) used to happen when our UnityPlayerActivity would receive onResume. (call stack will be shown below). Looking at the code of AppStartTrace.java at line 355 (at revision a350561 on April 25th, 2024 ), the line is:

353    if (isExperimentTTIDEnabled) {
354      View rootView = activity.findViewById(android.R.id.content);
355      rootView.getViewTreeObserver().addOnDrawListener(onDrawCounterListener);  // crash happens here

It is unclear why, in certain cases, line 354 would return null. The crash disappeared when we disabled ExperimentTTID by adding <meta-data android:name="experiment_app_start_ttid" android:value="false" /> to our AndroidManifest.xml

We have never reproduced the issue internally, or never noticed that it happened. It seems to happen on Android 12 and above.

Would a null check against rootView resolve the issue? Or is there a deeper reason and that value should never be null? Since our app is large, maybe the view hasn't finished initialization at that point? (Google Play vitals has flagged our Warm app start metric as problematic, if that helps.)

Steps to reproduce:

truncated call stack of crash:

Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewTreeObserver android.view.View.getViewTreeObserver()' on a null object reference
       at com.google.firebase.perf.metrics.AppStartTrace.onActivityResumed(AppStartTrace.java:355)
       at android.app.Application.dispatchActivityResumed(Application.java:436)
       at android.app.Activity.dispatchActivityResumed(Activity.java:1489)
       at android.app.Activity.onResume(Activity.java:2081)
       at com.unity3d.player.UnityPlayerActivity.onResume(UnityPlayerActivity.java:97)

firebase / firebase-android-sdk