概要

FirebaseCrashlytics The Crashlytics build ID is missing. 
This occurs when the Crashlytics Gradle plugin is missing from your app's build configuration. 
Please review the Firebase Crashlytics onboarding instructions at 
https://firebase.google.com/docs/crashlytics/get-started?platform=android#add-plugin

FATAL EXCEPTION: main
Process: com.XXXX.YYYY, PID: 29509
java.lang.RuntimeException: Unable to get provider com.google.firebase.provider.FirebaseInitProvider: java.lang.IllegalStateException: The Crashlytics build ID is missing. This occurs when the Crashlytics Gradle plugin is missing from your app's build configuration. Please review the Firebase Crashlytics onboarding instructions at https://firebase.google.com/docs/crashlytics/get-started?platform=android#add-plugin
    at android.app.ActivityThread.installProvider(ActivityThread.java:7807)
    at android.app.ActivityThread.installContentProviders(ActivityThread.java:7313)
    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:7020)
    at android.app.ActivityThread.-$$Nest$mhandleBindApplication(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2243)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loopOnce(Looper.java:205)
    at android.os.Looper.loop(Looper.java:294)
    at android.app.ActivityThread.main(ActivityThread.java:8214)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1187)
Caused by: java.lang.IllegalStateException: The Crashlytics build ID is missing. This occurs when the Crashlytics Gradle plugin is missing from your app's build configuration. Please review the Firebase Crashlytics onboarding instructions at https://firebase.google.com/docs/crashlytics/get-started?platform=android#add-plugin
    at com.google.firebase.crashlytics.internal.common.CrashlyticsCore.onPreExecute(CrashlyticsCore.java:145)
    at com.google.firebase.crashlytics.FirebaseCrashlytics.init(FirebaseCrashlytics.java:179)
    at com.google.firebase.crashlytics.CrashlyticsRegistrar.buildCrashlytics(CrashlyticsRegistrar.java:72)
    at com.google.firebase.crashlytics.CrashlyticsRegistrar.$r8$lambda$Pfd5XmDCFzNyAT9o9H6rDnTBQE4(Unknown Source:0)
    at com.google.firebase.crashlytics.CrashlyticsRegistrar$$ExternalSyntheticLambda0.create(Unknown Source:2)
    at com.google.firebase.tracing.ComponentMonitor.lambda$processRegistrar$0(ComponentMonitor.java:38)
    at com.google.firebase.tracing.ComponentMonitor$$ExternalSyntheticLambda0.create(Unknown Source:4)
    at com.google.firebase.components.ComponentRuntime.lambda$discoverComponents$0$com-google-firebase-components-ComponentRuntime(ComponentRuntime.java:160)
    at com.google.firebase.components.ComponentRuntime$$ExternalSyntheticLambda1.get(Unknown Source:4)
    at com.google.firebase.components.Lazy.get(Lazy.java:53)
    at com.google.firebase.components.ComponentRuntime.doInitializeEagerComponents(ComponentRuntime.java:322)
    at com.google.firebase.components.ComponentRuntime.initializeEagerComponents(ComponentRuntime.java:312)
    at com.google.firebase.FirebaseApp.initializeAllApis(FirebaseApp.java:607)
    at com.google.firebase.FirebaseApp.initializeApp(FirebaseApp.java:300)
    at com.google.firebase.FirebaseApp.initializeApp(FirebaseApp.java:264)
    at com.google.firebase.FirebaseApp.initializeApp(FirebaseApp.java:249)
    at com.google.firebase.provider.FirebaseInitProvider.onCreate(FirebaseInitProvider.java:69)
    at android.content.ContentProvider.attachInfo(ContentProvider.java:2619)
    at android.content.ContentProvider.attachInfo(ContentProvider.java:2589)
    at com.google.firebase.provider.FirebaseInitProvider.attachInfo(FirebaseInitProvider.java:61)
    at android.app.ActivityThread.installProvider(ActivityThread.java:7802)
    ... 11 more

Unity プロジェクトに Firebase Crashlytics を導入して Android Mono2x ビルドをしたら
アプリ起動直後に上記のエラーが発生してアプリが強制終了する現象に遭遇した

Android IL2CPP ビルドに切り替えたらエラーが発生しなくなった

その後、Android Mono2x ビルドに戻してもエラーが発生することはなかった