如何在文本框中加载Css并保存asp.net

Question

我希望用户在fckeditor中查看并将css保存在CMS中从哪里开始我试过

// aspx code
     <CKEditor:CKEditorControl runat="server" ID="txtckCss"   StartupMode="Source"></CKEditor:CKEditorControl><br>
//code behind
    string path = Server.MapPath("../App_Themes/White/Default.css");
    txtckCss.Text = System.IO.File.ReadAllText(path));

但是它给出了错误

Object reference not set to an instance of an object.