ScriptManager is required on the page to use ASP.NET AJAX Script Components

[nUflAvOrS]

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My page contains a scriptmanager and updatepanel.
What I would like to do is when user click on the checkbox within the gridview which is embeded inside a updatepanel. The gridview will changed the editindex into particular selected row index.

My code is here :

Protected Sub chkSelect_CheckedChanged(ByVal sender As Object, ByVal e As EventArgs)
        If IsPostBack Then
            Dim cb As CheckBox = DirectCast(sender, CheckBox)
            If cb IsNot Nothing Then
                If cb.Checked Then
                    Dim row As GridViewRow = DirectCast(cb.NamingContainer, GridViewRow)
                    Dim index As Integer = row.RowIndex
                    With grdListOfItem
                        .EditIndex = index
                        .DataSource = (CType(Session("myDatatable3"), DataTable)).DefaultView
                        .DataBind()
                    End With
                End If
            End If
        End If
    End Sub

<asp:TemplateField>
                <HeaderTemplate>
                    <asp:CheckBox ID="chkHeader" runat="server"  />
                </HeaderTemplate>
                <ItemTemplate>
                    <asp:CheckBox ID="chkSelect" runat="server"  AutoPostBack = "true" OnCheckedChanged = "chkSelect_CheckedChanged"/>
                </ItemTemplate>
            </asp:TemplateField>

But I get an error on .databind()
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System.InvalidOperationException was unhandled by user code Message="A ScriptManager is required on the page to use ASP.NET AJAX Script Components." Source="AjaxControlToolkit"

Hope someone can help me. Thanks a lot.