evaluateJavaScript of WKWebView always returns nil

Code Block ` swift
import Cocoa
import WebKit
let url = URL(string: "SOMEURL")!
let config = WKWebViewConfiguration()
let prefs = WKPreferences()
prefs.javaScriptEnabled = true
config.preferences = prefs
let webView = WKWebView(frame: .zero, configuration: config)
let navDel = NavDel()
webView.navigationDelegate = navDel
webView.load(URLRequest(url: url))
class NavDel: NSObject, WKNavigationDelegate {
  func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
    DispatchQueue.main.async {
      let js = "document.documentElement.textContent"
      webView.evaluateJavaScript(js) { (res, error) in
        guard error != nil else {
          print(error?.localizedDescription)
          return
        }
        print(res) // returns nil
      }
    }
  }
}