Converting local realms to synced realm - swift

Question

I want to convert my local realm database to a synced one to allow user authentication. I'm using swift and it wasn't included in the documentation, however, I found this method but it keeps giving SIGABRT exception and I don't know what seems to be the problem.
Here's what I added in the App Delegate:
import UIKit
import RealmSwift
import Realm
import Realm.Dynamic
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
let sourceFilePath = Bundle.main.url(forResource: "fieldFlow", withExtension: "realm")
let configuration = RLMRealmConfiguration()
configuration.fileURL = sourceFilePath
configuration.dynamic = true
configuration.readOnly = true
let localRealm = try! RLMRealm(configuration: configuration)
let creds = SyncCredentials.usernamePassword(username: "admin#realm.io", password: "password")
SyncUser.logIn(with: creds, server: URL(string: "http://localhost:9080")!) { (syncUser, error) in
DispatchQueue.main.async {
if let syncUser = syncUser {
self.copyToSyncRealmWithRealm(realm: localRealm, user: syncUser)
}
}
}
let config = Realm.Configuration(
// Set the new schema version. This must be greater than the previously used
// version (if you've never set a schema version before, the version is 0).
schemaVersion: 1,
// Set the block which will be called automatically when opening a Realm with
// a schema version lower than the one set above
migrationBlock: { migration, oldSchemaVersion in
// We haven’t migrated anything yet, so oldSchemaVersion == 0
if (oldSchemaVersion < 1) {
// Nothing to do!
// Realm will automatically detect new properties and removed properties
// And will update the schema on disk automatically
}
})
// Tell Realm to use this new configuration object for the default Realm
Realm.Configuration.defaultConfiguration = config
// Now that we've told Realm how to handle the schema change, opening the file
// will automatically perform the migration
let realm = try! Realm()
// Override point for customization after application launch.
return true
}
func copyToSyncRealmWithRealm(realm: RLMRealm, user: RLMSyncUser) {
let syncConfig = RLMRealmConfiguration()
syncConfig.syncConfiguration = RLMSyncConfiguration(user: user, realmURL: URL(string: "realm://localhost:9080/~/fieldRow")!)
syncConfig.customSchema = realm.schema
let syncRealm = try! RLMRealm(configuration: syncConfig)
syncRealm.schema = syncConfig.customSchema!
try! syncRealm.transaction {
let objectSchema = syncConfig.customSchema!.objectSchema
for schema in objectSchema {
let allObjects = realm.allObjects(schema.className)
for i in 0..<allObjects.count {
let object = allObjects[i]
RLMCreateObjectInRealmWithValue(syncRealm, schema.className, object, true)
}
}
}
}