呈现控件时出错,未将对象的引用设置到对象的实例(未創建子控件。。。)

///
        /// 将此控件呈现给指定的输出参数。
        ///
        /// 要写出到的 HTML 编写器
        protected override void Render(HtmlTextWriter output)
        {

            如果沒有下面一句，則報錯！！！

            EnsureChildControls();
            output.AddAttribute(HtmlTextWriterAttribute.Cellspacing, "0");
            output.AddAttribute(HtmlTextWriterAttribute.Cellpadding, "0");
            if (DesignMode)
            {
                output.AddAttribute(HtmlTextWriterAttribute.Style, "border:0px; border-collapse:collapse;");
            }
            output.RenderBeginTag(HtmlTextWriterTag.Table);

            output.RenderBeginTag(HtmlTextWriterTag.Tr);

            output.AddStyleAttribute(HtmlTextWriterStyle.BorderWidth, "0px");
            output.RenderBeginTag(HtmlTextWriterTag.Td);
            _hiddenField.RenderControl(output);
            _textBoxCode.RenderControl(output);
            output.RenderEndTag();

            output.AddStyleAttribute(HtmlTextWriterStyle.BorderWidth, "0px");
            output.RenderBeginTag(HtmlTextWriterTag.Td);
            output.AddAttribute(HtmlTextWriterAttribute.Style, "border:solid 1px #AECAF0; border-left:0px; border-right:0px; height:20px; line-height:20px;");
            output.AddAttribute(HtmlTextWriterAttribute.Type,"button");
            output.AddAttribute(HtmlTextWriterAttribute.Value,"選擇");
            output.AddAttribute(HtmlTextWriterAttribute.Onclick, "if(typeof(popupURL)!=\"undefined\"){popupURL.SetContentHTML('');popupURL.SetContentUrl('" + BaseDataUrl + "?t=" + BaseDataType + "&sid=" + FieldName + "&v='+(new Date()).getTime());popupURL.Show();}");
            output.RenderBeginTag(HtmlTextWriterTag.Input);
            output.RenderEndTag();
            output.RenderEndTag();

            output.AddStyleAttribute(HtmlTextWriterStyle.BorderWidth, "0px");
            output.RenderBeginTag(HtmlTextWriterTag.Td);
            _textBoxName.RenderControl(output);
            output.RenderEndTag();

            output.RenderEndTag();

            output.RenderEndTag();
        }