Getting object reference error in Gridview Hidden Field

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Gridview

ASP.NET4

Hi,

I am getting object reference error when i use Hidden field in gridview.

I bound the value in Hidden Controls. if value is nothing i am getting error.

I have checking Nothing also. But, still getting error in the code.

see my code

<asp:TemplateField HeaderText="Budget ManHours"  ItemStyle-Width="65%">
       <ItemTemplate>
              <asp:HiddenField ID="HdnSal" runat="server" Value='<%# Eval("Sal") %>' />
       </ItemTemplate>
       <ItemTemplate>
             <asp:TextBox ID="txtSal" runat="server" ForeColor="Blue" Text='<%# Eval("Sal") %>' Width="75%" ReadOnly="true" AutoPostBack="true" OnTextChanged="txtTot_TextChanged" MaxLength="7"></asp:TextBox>
       </ItemTemplate>
       <FooterTemplate>
              <asp:Label ID="lblTotal" runat="server" ForeColor="Green"></asp:Label>
       </FooterTemplate>
       <ItemStyle HorizontalAlign="Center"/>
</asp:TemplateField>

ASPX.VB Code
------------

Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound

       If e.Row.RowType = DataControlRowType.DataRow Then
           Dim chkSelect As CheckBox = TryCast(e.Row.FindControl("chkSelect"), CheckBox)
           Dim HdnSal As HiddenField = TryCast(e.Row.FindControl("HdnSal"), HiddenField)

           If HdnSal.Value = Nothing Then   //Error getting : Object reference if HdnSal is Nothing
               chkSelect.Checked = False
           Else
               chkSelect.Checked = True
           End If
       End If