嵌套的ASP.NET选项卡

I am trying to get a control tab inside another using ASP.NET and Ajax control toolkit , this is the code which i am using:

 <asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0">
     <asp:TabPanel runat="server" HeaderText="Main tab" ID="TabPanel1">
        <ContentTemplate>
                <asp:TabPanel HeaderText="Nested tab" ID="TabPanel2" runat="server">
                        <ContentTemplate>
                            <asp:Button ID="Button1" runat="server" Text="Delete" />
                        </ContentTemplate>
                </asp:TabPanel>            
        </ContentTemplate>
      </asp:TabPanel>
    </asp:TabContainer>

but i am getting an exception saying that :

"Object reference not set to an instance of an object."

I do not know what to do here, because my design needs to have a tab panel inside another one, is this possible ?

The nested <asp:TabPanel> should be in its own <asp:TabContainer>, that is probably what the exception is about: the object reference is probably the container, and it is not set to an instance since it's missing for the nested tab.

<asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0">
  <asp:TabPanel runat="server" HeaderText="Main tab" ID="TabPanel1">
    <ContentTemplate>

      <%-- You need another container before you can add a nested tab --%>
      <asp:TabContainer ID="NestedContainer1" runat="server">
        <asp:TabPanel HeaderText="Nested tab" ID="TabPanel2" runat="server">
          <ContentTemplate>
            <asp:Button ID="Button1" runat="server" Text="Delete" />
          </ContentTemplate>
        </asp:TabPanel>
      </asp:TabContainer>
      <%-- End of the second container --%>

    </ContentTemplate>
  </asp:TabPanel>
</asp:TabContainer>