[Solved] ASP.Net Error: A control with ID chkAll could not be found for the trigger in UpdatePanel

Getting Error :

A control with ID 'chkAll' could not be found for the trigger in UpdatePanel 'UpdatePanel1'.

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true"></asp:ScriptManager>
<asp:updatepanel id="UpdatePanel1" runat="server" >
   <contenttemplate>
      <asp:GridView ID="gw_poitem" runat="server" AutoGenerateColumns="false" OnRowDataBound = "OnRowDataBound" DataKeyNames="item"  CssClass="table table-bordered table-striped"  Style="white-space: nowrap; font-size: 12px; letter-spacing: 0.50px;" >
         <Columns>
            <asp:TemplateField HeaderText="#"  HeaderStyle-Width="20px">
               <ItemTemplate><%# Container.DataItemIndex + 1 %></ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField  HeaderStyle-Width="10px" >
               <HeaderTemplate>
                  <asp:CheckBox ID="chkAll" runat="server" AutoPostBack="true" OnCheckedChanged="OnCheckedChanged" />
               </HeaderTemplate>
               <ItemTemplate>
                  <asp:CheckBox   ID="chkDel" runat="server" AutoPostBack="true" OnCheckedChanged="OnCheckedChanged" />
               </ItemTemplate>
            </asp:TemplateField>            
            <asp:TemplateField HeaderText="Requistion No"  HeaderStyle-Font-Bold="true" HeaderStyle-Width="100px"  >
               <ItemTemplate>
                  <asp:Label runat="server" ID="lblreqno" Text='<%# Eval("reqno") %>'></asp:Label>
                  <asp:TextBox ID="txtreqno" ReadOnly="true" style="white-space:nowrap"  CssClass="form-control" runat="server" Text='<%# Eval("reqno") %>' Visible="false"></asp:TextBox>
               </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Item Code" HeaderStyle-Font-Bold="true"  HeaderStyle-Width="100px">
               <ItemTemplate>
                  <asp:Label runat="server" ID="lblitem"  Text='<%# Eval("item") %>'></asp:Label>
                  <asp:TextBox ID="txtitem" style="width:100px; white-space:nowrap"  CssClass="form-control" runat="server" Text='<%# Eval("item") %>' Visible="false"></asp:TextBox>
               </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Description" HeaderStyle-Font-Bold="true" HeaderStyle-Width="300px" >
               <ItemTemplate>
                  <asp:Label  runat="server" ID="lbldsca" Text='<%# Eval("dsca") %>'></asp:Label>
                  <asp:TextBox ID="txtdsca" ReadOnly="true"  style="white-space:nowrap"  CssClass="form-control" runat="server" Text='<%# Eval("dsca") %>' Visible="false"></asp:TextBox>
               </ItemTemplate>
            </asp:TemplateField>                     
         </Columns>
      </asp:GridView>
   </ContentTemplate>
   <Triggers>
      <asp:AsyncPostBackTrigger ControlID="chkall" EventName="CheckedChanged" />
   </Triggers>
</asp:UpdatePanel>