last controller's content visible while pushing a new view controller

I am facing a very basic problem while pushing a view controller. When I am pushing a new controller, the last controller's content is visible on screen. I am pushing the new controller as follows:

DispatchQueue.main.async {
                    self.navigationController?.pushViewController(reviewFeatureViewController, animated: true)
                }

Copyright Notice：Content Author:「pkc456」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/56795553/last-controllers-content-visible-while-pushing-a-new-view-controller