Get Android Token in Xamarin to send notification with Azure Hub

In an Xamarin Forms project under Visual Studio 2017, I can't get the FCM (Firebase Cloud Messaging) token for Android devices (.Droid project) to register it to FCM.

I created an application on the Firebase console, and retrieved the ApplicationId, ApiKey, ProjectId and GcmSenderId.

Here is the code I use in the .Droid project on Xamarin.Forms:

var options = new FirebaseOptions.Builder()
                    .SetApplicationId("xxxx")
                    .SetApiKey("xxxxxxxxxxxxx") 
                    .SetProjectId("xxxxxxxxx")
                    .SetDatabaseUrl("xxxxxxxxxxxxxxx")
                    .SetGcmSenderId("xxxxxxxxxxxx")
                    .Build();

FirebaseApp app = FirebaseApp.InitializeApp(Android.App.Application.Context, options);
String token = FirebaseInstanceId.Instance.Token; // RETURNS NULL
String authorizedEntity = "xxxxxxxxxx";
String scope = "GCM";
String token2 = FirebaseInstanceId.Instance.GetToken(authorizedEntity, scope); // ALSO RETURNS NULL

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I do not understand why the GetToken () function returns NULL.
Can you help me please ?

For the UWP project, I can recover it by the following method:
var channel = await PushNotificationChannelManager.CreatePushNotificationChannelForApplicationAsync ();
String Token = channel.Uri;

I followed this doc:
https://docs.microsoft.com/fr-fr/azure/app-service-mobile/app-service-mobile-xamarin-forms-get-started-push