FirebaseUI-iOS: swift 3, iOS Firebase Database sometimes crash when writing data (set value)

sometimes my app is crashing when I´m trying to set a value in a Firebase Database. Here´s my code:

var databaseRef = FIRDatabaseReference()

func sendregistrationtoken() {
    print("tokenRefreshNotification")

    guard let contents = FIRInstanceID.instanceID().token()
        else {
            return
    }

    if let userusername: String = Keychain().accessibility(.afterFirstUnlock)["userusername"] {
        if let refreshedToken = FIRInstanceID.instanceID().token(){
            let defaults = UserDefaults.standard
            defaults.set(refreshedToken, forKey: "userregistrationtoken")
            let userid = userusername.replacingOccurrences(of: ".", with: "%2E")
            let post : [String:AnyObject] = ["token" : refreshedToken as AnyObject]

            databaseRef = FIRDatabase.database().reference()
            databaseRef.child("Users").child(userid.uppercased()).setValue(post){ (error, ref) -> Void in
                print(error ?? "noerror")
            }

            connectToFcm()
        }
    }

}

func connectToFcm() {
    print("connectToFcm")
    FIRMessaging.messaging().connect { (error) in
        if (error != nil) {
            print("Unable to connect with FCM. \(error)")
        } else {
            print("Connected to FCM.")
        }
    }
}

I call sendregistrationtoken() in my viewdidload of may main view controller. Here´s what my crash report says:

+[FValidation validateFrom:validPathString]

Many Thanks Stephan