getElementById即使在页面加载后也返回null-WkWebView和Swift

https://app.oncoursesystems.com/homeworkportal/index/brrsd

https://app.oncoursesystems.com/homeworkportal/index/brrsd ,NSLocalizedDescription=JavaScript异常发生})

func网络视图:didFinishNavigation: (使用WKNavigationDelegate)但我遇到了相同的错误

let js = "var myelement = document.getElementById(\"Username\").value='xxx'; myelement.innerHTML= \"New Text\";"
    webView.evaluateJavaScript(js, completionHandler: nil)

webView.evaluateJavaScript("document.getElementById('Username').value='xxx'", completionHandler: nil)

webView.evaluateJavaScript("window.onload = function() { alert(document.getElementById('Username').value='xxx');};", nil)

//I got this error instead for the last one: Error: Optional(Error Domain=WKErrorDomain Code=5 "JavaScript execution returned a result of an unsupported type" UserInfo={NSLocalizedDescription=JavaScript execution returned a result of an unsupported type})

我在viewDidLoad、webView:didFinishNavigation:(确保html已加载)和单击按钮上尝试了所有这些

我该怎么做才能成功地用值“xxx”填充表单的用户名字段?这是由链接本身的问题引起的还是我的代码有什么问题?