final setup medium webview

vc.swift

override func loadView() {
  super.loadView()
  
  let contentController = WKUserContentController()
  contentController.add(self, name: "callback")
  
  let config = WKWebViewConfiguration()
  config.userContentController = contentController
  
  webview = WKWebView(frame: webview.frame, configuration: config)
}