asp.net – 我们如何使用通用处理程序加载usercontrol？

<script type="text/javascript">
 function fillSigns() { 
                $.ajax({
                    url: "usercontrolhandler.ashx?control=signs.ascx",context: document.body,success: function (data) {                       
                        $('#signdiv').html(data);
                    }
                });
            }  
 </script>

这是处理程序文件中的代码

public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        Page page = new Page();
        UserControl ctrl = (UserControl)page.LoadControl("~/" + context.Request["control"] + ".ascx");      
        page.Form.Controls.Add(ctrl);

        StringWriter stringWriter = new StringWriter();
        HtmlTextWriter tw = new HtmlTextWriter(stringWriter);
        ctrl.RenderControl(tw);
        context.Response.Write(stringWriter.ToString());
    }

此代码在下面显示的行中引发对象引用未找到错误.

page.Form.Controls.Add(ctrl);