在页面加载中动态添加css时出错

Question

我正在尝试动态添加一些css，但它返回错误可以anyOne帮助或任何其他更好的方法来实现这一点我还需要实现一些js。

protected override void OnInit(EventArgs e)
{
    XmlDocument doc = new XmlDocument();
    string path = Server.MapPath("~/templates/1/XMLFile.xml");
    doc.Load(path);
    XmlNodeList nodes = doc.DocumentElement.SelectNodes("/template/theme");
    List<css> books = new List<css>();

    foreach (XmlNode node in nodes)
    {
        css book = new css();

        book.name1 = node.SelectSingleNode("css/name1").InnerText;
        book.name2 = node.SelectSingleNode("css/name2").InnerText;
        book.name3 = node.SelectSingleNode("css/name3").InnerText;
        book.name4 = node.SelectSingleNode("css/name4").InnerText;

//error occured in below line
        this.Header.InnerHtml += this.Header.InnerHtml + "<link type=\"text/css\" rel=\"Stylesheet\" href=\"'"+book.name1+"'\" />";
        this.Header.InnerHtml += this.Header.InnerHtml + "<link type=\"text/css\" rel=\"Stylesheet\" href=\"'" + book.name2 + "'\" />";
        this.Header.InnerHtml += this.Header.InnerHtml + "<link type=\"text/css\" rel=\"Stylesheet\" href=\"'" + book.name3 + "'\" />";
        this.Header.InnerHtml += this.Header.InnerHtml + "<link type=\"text/css\"  rel=\"Stylesheet\" href=\"'" + book.name4 + "'\" />";

    }
}
 class css
{
public string name1;
public string name2;
public string name3;
public string name4;
}

Error-System.NullReferenceException
{"Object reference not set to an instance of an object."}