Fido app: Missing or invalid authentication

When starting the Fido app you can sign in using a Google account and afterwards logcat shows the following error
```
GAEService: Error extracting information from server's registration request
GAEService: com.google.api.client.googleapis.json.GoogleJsonResponseException: 503 Service Unavailable
GAEService: GET https://webauthndemo.appspot.com/_ah/api/fido2RequestHandler/v1/get/register
GAEService: {
GAEService: "code": 503,
GAEService: "errors": [
GAEService: {
GAEService: "domain": "global",
GAEService: "message": "com.google.appengine.api.datastore.DatastoreFailureException: Missing or invalid authentication.",
GAEService: "reason": "backendError"
GAEService: }
GAEService: ],
GAEService: "message": "com.google.appengine.api.datastore.DatastoreFailureException: Missing or invalid authentication."
GAEService: }
GAEService: at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:113)
GAEService: at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:40)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.interceptResponse(AbstractGoogleClientRequest.java:321)
GAEService: at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1111)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:419)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:352)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:469)
GAEService: at com.fido.example.fido2apiexample.GAEService.getRegistrationRequest(GAEService.java:105)
GAEService: at com.fido.example.fido2apiexample.Fido2DemoActivity$11.call(Fido2DemoActivity.java:472)
GAEService: at com.fido.example.fido2apiexample.Fido2DemoActivity$11.call(Fido2DemoActivity.java:468)
GAEService: at com.google.android.gms.tasks.zzy.run(com.google.android.gms:play-services-tasks@@17.2.0:2)
GAEService: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
GAEService: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
GAEService: at java.lang.Thread.run(Thread.java:923)
Fido2DemoActivity: Register request is null
```

Le chemin d’échec est indiqué dans GAEService.java:105 et Fido2DemoActivity.java:468-472 ; commencez par reproduire le flux de connexion et examiner l’appel de demande d’enregistrement ainsi que son résultat null. C’est terminé lorsque l’application Fido obtient une demande d’enregistrement valide après la connexion à Google, sans la sortie 503 ni « Register request is null » consignées.

Rédigé par le modèle d'indexation à partir du texte de l'issue.