无法调用方法＆＃34; appendChild＆＃34;

Question

我正在使用HtmlUnit，而我目前正在尝试登录网站。问题是，当我运行代码时，我收到一个指向我的getPage(...)行（下面的代码）的巨大错误，它有错误：

net.sourceforge.htmlunit.corejs.javascript.EcmaError: TypeError: 
Cannot call method "appendChild" of undefined (script in 
http://bukkit.org/login from (77, 13) to (85, 14)#84)' I have just 
tried to use the code to login. It is just like the example code on 
the website.

代码：

try(final WebClient web = new WebClient()) {
    final HtmlPage login = web.getPage("http://bukkit.org/login");
    final HtmlForm form = (HtmlForm) login.getElementById("pageLogin");

    String username;
    String password;

    print("Please enter your username: ");
    username = scanner.nextLine();

    print("Please enter your password: ");
    password = scanner.nextLine();

    final HtmlTextInput name = form.getInputByName("login");
    name.setValueAttribute(username);
    final HtmlPasswordInput pswd = form.getInputByName("password");
    pswd.setValueAttribute(password);

    final HtmlSubmitInput button = form.getInputByValue("Log in");
    button.click();
}

导致错误的行是

final HtmlPage login = web.getPage("http://bukkit.org/login");