xml错误:对象引用在SelectSingleNode之后没有设置为对象的实例

here's my code:

这是我的代码:

XmlDocument doc = new XmlDocument();
foreach (string c in colorList)
{
     doc.Load(@"https://whoisxmlapi.com/whoisserver/WhoisService?domainName=" + c + @"&username=user&password=pass");
     textBox1.Text += doc.SelectSingleNode("WhoisRecord/registrant/email").InnerText + ",";
}
XmlDocument doc = n