coredata update json background thread problem

Hi,
I have the following coredata singleton class
in my appdelegate i try to update the data from json
but in my app when it launches i get different error messages regarding thread or observer of NSManagedObjectContextObjectsDidChangeNotification
or Incorrect guard value: or Main Thread Checker: UI API called
what is the problem and what changes should i make for it to work?
thanks

Code Block 
import CoreData
import Foundation
class CoreDataStack {
   
  static let sharedManager = CoreDataStack()
  private init() {} // Prevent clients from creating another instance.
   
  //This is the name of your coreData Database
  static let modelName = "myDB"
  static let FirstLaunchKey = "firstLaunch"
 
  lazy var managedObjectModel: NSManagedObjectModel = {
    let modelURL = Bundle.main.url(forResource: CoreDataStack.modelName, withExtension: "momd")!
    return NSManagedObjectModel(contentsOf: modelURL)!
  }()
   
   
  lazy var applicationDocumentsDirectory: URL = {
    return NSPersistentContainer.defaultDirectoryURL()
  }()
   
  lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator = {
    let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
     
    let persistentStoreURL = self.applicationDocumentsDirectory.appendingPathComponent(CoreDataStack.modelName + ".sqlite")
     
    do {
       
      // let dict = [NSSQLitePragmasOption: ["journal_mode":"DELETE"]]
  try coordinator.addPersistentStore(ofType: NSSQLiteStoreType,
                    configurationName: nil,
                    at: persistentStoreURL,
              options: [NSMigratePersistentStoresAutomaticallyOption: true,
                   NSInferMappingModelAutomaticallyOption: false])
    } catch {
      fatalError("Persistent store error! \(error)")
    }
     
    return coordinator
  }()
   
   
  fileprivate lazy var saveManagedObjectContext: NSManagedObjectContext = {
    let moc = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
    moc.persistentStoreCoordinator = self.persistentStoreCoordinator
    return moc
  }()
   
  @objc lazy var mainObjectContext: NSManagedObjectContext = {
    let managedObjectContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
    managedObjectContext.parent = self.saveManagedObjectContext
    return managedObjectContext
  }()
   
  func saveMainContext() {
    guard mainObjectContext.hasChanges || saveManagedObjectContext.hasChanges else {
      return
    }
 
    mainObjectContext.performAndWait() {
      // print("save performAndWait")
      do {
        try self.mainObjectContext.save()
      } catch {
        fatalError("Error saving main managed object context! \(error)")
      }
    }
    saveManagedObjectContext.perform() {
      // print("save simple")
      do {
        try self.saveManagedObjectContext.save()
      } catch {
        fatalError("Error saving private managed object context! \(error)")
      }
    }
  }
  func removeDatabaseForVersion(version:String){
    let previouslyVersion = UserDefaults.standard.bool(forKey: version)
     if !previouslyVersion {
      UserDefaults.standard.set(true, forKey: version)
      // Default directory where the CoreDataStack will store its files
      let directory = NSPersistentContainer.defaultDirectoryURL()
      let url = directory.appendingPathComponent(CoreDataStack.modelName + ".sqlite")
       
      let shmURL = directory.appendingPathComponent(CoreDataStack.modelName + ".sqlite-shm")
       
      let walURL = directory.appendingPathComponent(CoreDataStack.modelName + ".sqlite-wal")
       
      _ = try? FileManager.default.removeItem(at: url)
      _ = try? FileManager.default.removeItem(at: shmURL)
      _ = try? FileManager.default.removeItem(at: walURL)
    }
  }
   
 
}
 
[update db class](https://developer.apple.com/forums/content/attachment/31e8ff0d-abb3-4fb7-a346-c05f8a610b23){: .log-attachment}

in my appdelegate:

Code Block   
UpdateDbClass.updateDatabase(entityName: DbTable.VehiclesEntity.rawValue, completionHandler: {
      print(" DB updated delegate")
    })