DispatchQueue : Cannot be called with asCopy = NO on non-main thread

You should call all code from showAlertMessage on main queue:

class func showAlertMessage(message:String, viewController: UIViewController) {
    DispatchQueue.main.async {
        let alertMessage = UIAlertController(title: "", message: message, preferredStyle: .alert)

        let cancelAction = UIAlertAction(title: "Ok", style: .cancel)

        alertMessage.addAction(cancelAction)

        viewController.present(alertMessage, animated: true, completion: nil)
    }
}