bind a grid view using session in the where clause

i have created a grid view in c#:

<asp:GridView ID="GridView1" runat="server" AllowPaging="True"
                               AutoGenerateColumns="False" DataKeyNames="Id"
                                >
                               <Columns>
                                   <asp:BoundField DataField="Id" HeaderText="Id" InsertVisible="False"
                                       ReadOnly="True" SortExpression="Id" />
                                   <asp:BoundField DataField="HeaderId" HeaderText="HeaderId"
                                       SortExpression="HeaderId" />
                                  
                                   <asp:BoundField DataField="Description" HeaderText="Description"
                                       SortExpression="Description" />
                                   <asp:BoundField DataField="DeviceId" HeaderText="DeviceId"
                                       SortExpression="DeviceId" />
                                   <asp:BoundField DataField="DateStamp" HeaderText="DateStamp"
                                       SortExpression="DateStamp" />
                                   
                               </Columns>
                           </asp:GridView>
                           <asp:SqlDataSource ID="SqlDataSource3" runat="server"
                               ConnectionString="<%$ ConnectionStrings:ConnectionStringline %>"
                               SelectCommand="SELECT * FROM [Tables]">
                               <SelectParameters>
                                   <asp:SessionParameter Name="HeaderId" SessionField="id" Type="Int32" />
                               </SelectParameters>
                           </asp:SqlDataSource>

and in the page apsx.cs
i have bind this gridview

public void bindgridview()
       {

           SqlCommand sqlcmd;
           SqlDataAdapter dap;
           DataTable data = new DataTable();
           SqlConnection sqlcon = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionStringline"].ConnectionString);

           sqlcon.Open();

           sqlcmd = new SqlCommand("SELECT [Id], [HeaderId],[Description], [DeviceId], [DateStamp],  FROM [Tables] WHERE ([HeaderId] = '"+Session["id"].ToString()+"')", sqlcon);

           dap = new SqlDataAdapter(sqlcmd);

           dap.Fill(data);
           if (data.Rows.Count > 0)
           {
               GridView1.DataSource = data;

               GridView1.DataBind();
           }
       }

BUT WHEN I RUN GIVES ME THIS ERROR:

Object reference not set to an instance of an object.


WHAT TO FIX IN THIS CASE?

Tags: C# (C# 4.0), ASP.NET