Fido app: Missing or invalid authentication

[jpstotz]

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