title: popover is not working in swift 4
body: <p>I am using UIPopoverPresentationControllerDelegate to show my popover, it's working fine but not able to manage width and height of the popover viewController. And how to set Presentation over current context ??</p>
  
  <pre><code>func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
  
      let showObjectVC = storyboard?.instantiateViewController(withIdentifier: "ShowActorDetail") as! ShowActorDetailsViewController
      showObjectVC.modalPresentationStyle = .popover
  
      showObjectVC.popoverPresentationController?.sourceView = tableView.cellForRow(at: indexPath)
      showObjectVC.popoverPresentationController?.delegate = self
      showObjectVC.preferredContentSize = CGSize(width: 400, height: 300)
  
      present(showObjectVC, animated: true, completion: nil)
  }
  </code></pre>