WKWebView UserContentController notworking

Hi, can someone tell me what is wrong? The WebView working fine, but the JS to Native is not triggered/working/called.

let contentController = WKUserContentController()

            contentController.add(self, name: "callbackHandler")

            webView.configuration.userContentController = contentController
func userContentController(_ UserContentController: WKUserContentController, didReceive message: WKScriptMessage) {

        print(message.name)

        if (message.name == "callbackHandler"){

                    print("\(message.body)")

                }

    }

My JS:

window.webkit.messageHandlers.callbackHandler.postMessage("Hello from JavaScript");
WKWebView UserContentController notworking
 
 
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