项目里更换完google-services.json文件后，获取 firebase token 时，显示报错：
 

E/FirebaseInstanceId: Topic sync or token retrieval failed on hard failure exceptions: FIS_AUTH_ERROR. Won't retry the operation.D/AndroidRuntime: Shutting down VMcom.google.android.gms.tasks.RuntimeExecutionException: java.io.IOException: FIS_AUTH_ERRORat com.google.android.gms.tasks.zzu.getResult(Unknown Source:15)at com.myApp.MainActivity$2.onComplete(MainActivity.java:349)at com.google.android.gms.tasks.zzj.run(Unknown Source:4)at android.os.Handler.handleCallback(Handler.java:883)at android.os.Handler.dispatchMessage(Handler.java:100)at android.os.Looper.loop(Looper.java:237)at android.app.ActivityThread.main(ActivityThread.java:7804)at java.lang.reflect.Method.invoke(Native Method)at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1068)Caused by: java.io.IOException: FIS_AUTH_ERRORat com.google.firebase.iid.zzs.zza(com.google.firebase:firebase-iid@@20.1.0:82)at com.google.firebase.iid.zzs.zza(com.google.firebase:firebase-iid@@20.1.0:96)at com.google.firebase.iid.zzx.then(com.google.firebase:firebase-iid@@20.1.0:4)at com.google.android.gms.tasks.zzd.run(Unknown Source:5)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)at java.lang.Thread.run(Thread.java:919)

 对我有用的办法：
删除build.gradle文件，clear 和 rebuild