断言失败-[UIAlertController addTextFieldWithConfigurationHandler

在WKWebView中调用JavaScript中的window.prompt会产生断言错误:

断言失败-[UIAlertController .addTextFieldWithConfigurationHandler:]

断言错误来自这个WKUIDelegate函数:

func webView(_ webView: WKWebView, runJavaScriptTextInputPanelWithPrompt prompt: String, defaultText: String?, initiatedByFrame frame: WKFrameInfo,
             completionHandler: @escaping (String?) -> Void) {
    let alertController = UIAlertController(title: nil, message: prompt, preferredStyle: .actionSheet)
    alertController.addTextField { (textField) in
        textField.text = defaultText
    }
    alertController.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in
        if let text = alertController.textFields?.first?.text {
            completionHandler(text)
        } else {
            completionHandler(defaultText)
        }
    }))
    alertController.addAction(UIAlertAction(title: "Cancel", style: .default, handler: { (action) in
        completionHandler(nil)
    }))
    present(alertController, animated: true, completion: nil)
}

类文档没有显示在添加文本字段或初始化项时添加配置处理程序的方法。那你该怎么处理这件事?