Flutter Firestore : getToken aborted due to token change

I'm trying to fetch a data from Firestore, and I got this error message that I don't really understand :

W/Firestore( 4463): (21.3.0) [FirestoreCallCredentials]: Failed to get token: com.google.firebase.firestore.FirebaseFirestoreException: getToken aborted due to token change.

  final userCollection = Firestore.instance.collection('users');

  Future<DocumentSnapshot> getUser(String id){
    return userCollection.document(id).get();
  }

I tried to google it but there's no clear answer. It seems to be linked with the SHA-1 fingerprint, which is weird 'cause I set up firestore according to the official doc so I don't get it :/