I have tested again and find out that it will get the error above in design mode. However, the code works well when run it. In my opinion, it is a bug about VS
please find my code below
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" PageSize="6"
Height="1173px" Width="705px" DataSourceID="AccessDataSource1">
<Columns>
<asp:BoundField DataField="ID" HeaderText="ID" InsertVisible="False" ReadOnly="True"
SortExpression="ID" />
<asp:BoundField DataField="Content" HeaderText="Content" SortExpression="Content" />
<asp:TemplateField>
<ItemTemplate>
<asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="1">
<asp:TabPanel ID="TabbPnl1" runat="server">
<HeaderTemplate>
View User Trace
</HeaderTemplate>
<ContentTemplate>
<asp:Panel ID="pnlMain" runat="server">
<asp:TextBox ID="txt" runat="server" Text="panel1"></asp:TextBox></asp:Panel>
</ContentTemplate>
</asp:TabPanel>
<asp:TabPanel ID="TabPanel1" runat="server">
<HeaderTemplate>
View User Trace2
</HeaderTemplate>
<ContentTemplate>
<asp:Panel ID="Panel1" runat="server">
<asp:TextBox ID="TextBox1" runat="server" Text="panel2"></asp:TextBox>
</asp:Panel>
</ContentTemplate>
</asp:TabPanel>
</asp:TabContainer>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
<asp:AccessDataSource ID="AccessDataSource1" runat="server" DataFile="~/App_Data/Database1.accdb"
SelectCommand="SELECT * FROM [Table1]"></asp:AccessDataSource>
</div>
</form>
</body>
</html>