validation in chekboxes in repeater

'get how many packages have been selected         
Dim c As Int32
         Dim str As String
         For Each dataItem As RepeaterItem In Repeater1.Items             
Dim ckchecked As CheckBox = DirectCast(dataItem.FindControl("chk_packages"), CheckBox)             If ckchecked.Checked Then
                 
Dim chk_single As CheckBox = DirectCast(dataItem.FindControl("single_rate"), CheckBox)                 Dim chk_double As CheckBox = DirectCast(dataItem.FindControl("double_rate"), CheckBox)                 If chk_single.Checked Then
                     c = c + 1
                 ElseIf chk_double.Checked Then
                     c = c + 1
                 End If
             End If
         Next
         If c = 0 Then
             lbl_error_package.Visible = True
             lbl_error_package.Text = "please select at least one package"
         Else
             lbl_error_package.Visible = False
         End If

<asp:Repeater ID="Repeater1"  runat="server">
             <HeaderTemplate>
<table border="1" width="100%">
<tr>
<th>id</th>
<th align=left>Room Categories</th>
<th>single</th>
<th>Double</th>
</tr>
</HeaderTemplate>
<ItemTemplate>
<tr>
<td width="5%">
    <asp:Label ID="Label3" runat="server" Text= '<%# Container.DataItem("packageid")%>' /></td>
<td> <asp:CheckBox ID="chk_packages" AutoPostBack=true runat="server" Text = <%#Container.DataItem("package_name")%> />  </td>
<td><%#Container.DataItem("single_rate")%><asp:CheckBox  AutoPostBack=true ID="single" Enabled=false runat="server" />Number of Rooms<asp:TextBox ID="txt_single" Enabled=false  width="10px"   MaxLength="2" runat="server"></asp:TextBox></td>
<td><%#Container.DataItem("double_rate")%><asp:CheckBox  AutoPostBack=true ID="Double" Enabled=false runat="server" />Number of Rooms<asp:TextBox ID="txt_double" Enabled =false  width="10px"   MaxLength="2" runat="server"></asp:TextBox></td>
<td></td>
</tr>
</ItemTemplate>
<FooterTemplate>
</table>
</FooterTemplate>
         </asp:Repeater>

its unable to find control and give exception
Object reference not set to an instance of an object.