Realm Swift: retrieves the user, the object does not return when the application starts

Question

I use Realm to store user information. When I launch the application, I scan the Realm database to see if there is a user object (to determine if the user is logged in or logged out). Here is my code:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    let config=Realm.Configuration(
        schemaVersion: 2,
        migrationBlock: { migration, oldSchemaVersion in
            if (oldSchemaVersion<1) || (oldSchemaVersion<2){
                migration.enumerateObjects(ofType: User.className()) { oldObject, newObject in
                }
            }
    })

    Realm.Configuration.defaultConfiguration=config

    do{
        let r=try Realm()
        print(Realm.Configuration.defaultConfiguration.fileURL!.path)
        let u=r.objects(User.self)
        //prints 0
        print("user:", u.count)
    }catch let e as NSError{
        print("failed to initate realm /", e)
    }

    return true
}

I can see from the Realm browser that the user is in the database . But I can’t understand it. I probably initiate Realm wrong, but I can't figure it out. Any idea?

---

UPDATE : Here is the code that I call to save the user during registration or registration:

DispatchQueue.main.async{
    do{
        let r=try Realm()
        try r.write{
            let u=User(id:"id", username:"username")
                r.add(u)
             }
         }catch let e as NSError{
             print(e)
         }
     }
}