关于repeater控件的一个常见问题

前台页面:
<asp:repeater id="rptNews" runat="server">
<ItemTemplate>
<div id="main_Content_title">
<%# DataBinder.Eval(Container.DataItem,"title")%>
</div>
<div id="main_Content_content">
<%# DataBinder.Eval(Container.DataItem,"content")%>
<asp:Panel id="PanelReply" runat="server">
<div id="reply">
<div id="gbook_reply_title">
<%# DataBinder.Eval(Container.DataItem,"replyName")%>
</div>
<div id="reply_content">
<%# DataBinder.Eval(Container.DataItem,"replyContent")%>
</div>
</div>
</asp:Panel>
</div>
</ItemTemplate>
</asp:repeater>
后台页面:
private void rptNews_ItemDataBound(object sender, System.Web.UI.WebControls.RepeaterItemEventArgs e)
{
DataRowView rv = (DataRowView)e.Item.DataItem ;
string replyName = rv["replyname"].ToString();
if (replyName!="") 
{
this.PanelReply.Visible=true;
}
else
{
this.PanelReply.Visible=false;
}
}初学asp.net,请问为何总是显示"未将对象引用设置到对象的实例",错在哪儿呢?请指教!!!