为什么Firebase在Google Play验证成功后返回“null”帐户？

class MyActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        [...]
        val activity = this
        val gso = GoogleSignInOptions
            .Builder(GoogleSignInOptions.DEFAULT_GAMES_SIGN_IN)
            .requestServerAuthCode("<some-id>.apps.googleusercontent.com")
            .build()
        val signInClient = GoogleSignIn.getClient(activity, gso)
        val acc = GoogleSignIn.getLastSignedInAccount(activity) // This is null
        val currentUser = Firebase.auth.currentUser // This is null
        signInClient.silentSignIn().addOnCompleteListener(activity) { task ->
            if (task.isSuccessful) {
                val account = task.result // This is com.google.android.gms.auth.api.signin.GoogleSignInAccount@123456
                account.displayName // This is null
                account.id // This is null
                Firebase.auth.currentUser // Still null
            }
        }
    }
}

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根据文档，如果“isSuccessful”被触发，则用户ID不应为“null”：https://firebase.google.com/docs/auth/android/play-games