【Unity】FirebaseCrashlytics The Crashlytics build ID is missing.

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