how to Update columns having NULL values in Gridview?

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C#

ASP.NET

i have a gridview with 6 columns..while inserting data into gridview i fill first 4 columns means last 2 columns Name and Size allow NULL values. Later while updating the gridview i want to add data to last 2 columns. since they are null im getting error.i tried to solve this but getting Object reference not set to an instance of an object
below is the code..thanks in advc!!

protected void UpdateCustomer(object sender, GridViewUpdateEventArgs e)
   {

       SqlConnection con = new SqlConnection("Data Source=VAIO-VAIO;Initial Catalog=CMS;Integrated Security=True");

       string name = GridView2.DataKeys[e.RowIndex].Values["Name"].ToString();
       TextBox txtName = (TextBox)GridView2.Rows[e.RowIndex].FindControl("txtName");
       string size = GridView2.DataKeys[e.RowIndex].Values["Size"].ToString();
       TextBox txtSize = (TextBox)GridView2.Rows[e.RowIndex].FindControl("txtSize");


           con.Open();
           SqlCommand cmd = new SqlCommand("update Tab2 set Name='" + txtName + "' and  Size= '" + txtSize + "' ", con);
           cmd.ExecuteNonQuery();
           con.Close();
           //lblresult.ForeColor = Color.Green;
           //lblresult.Text = sitename + " Details Updated successfully";
           GridView2.EditIndex = -1;
           GridView2.DataBind();
   }

<asp:GridView ID="GridView2" runat="server" AutoGenerateColumns="False" 
     Width="1132px" AllowPaging="True" 
     onrowediting="EditCustomer"
onrowupdating="UpdateCustomer"  onrowcancelingedit="CancelEdit"
      
      DataKeyNames="name, size"  >
     <PagerStyle ForeColor="Black" HorizontalAlign="Center" 
               BackColor="#C6C3C6"></PagerStyle>




        <Columns>
         
          <%-- <asp:TemplateField HeaderText="Stockcat">
                                <ItemTemplate>
                                    <asp:DropDownList ID="DropDownList1" runat="server" 
                                     AutoPostBack="True">
                                   
                                    <asp:ListItem Value="des" Text="des" />
                                    <asp:ListItem Value="bms" Text="bms" />
                                   
                                    </asp:DropDownList>

</ItemTemplate>  

</asp:TemplateField> --%> 
                   <asp:BoundField DataField="Stockcat" HeaderText="Stockcat" 
                SortExpression="Stockcat" />
           

            <asp:BoundField DataField="Stockid" HeaderText="Stockid" 
                SortExpression="Stockid" />
           
           
            <asp:BoundField DataField="Company" HeaderText="Company" 
                SortExpression="Company" />
                 <asp:BoundField DataField="Date" HeaderText="Date" SortExpression="Date" />

           <asp:TemplateField ItemStyle-Width = "150px"  HeaderText = "Name">
    <ItemTemplate>
        <asp:Label ID="lblName" runat="server"
            Text='<%# Eval("Name")%>'></asp:Label>
    </ItemTemplate>
    <EditItemTemplate>
        <asp:TextBox ID="txtName" runat="server"
            Text='<%# Eval("Name")%>'></asp:TextBox>
    </EditItemTemplate> 
    <FooterTemplate>
        <asp:TextBox ID="txtName" runat="server"></asp:TextBox>
    </FooterTemplate>
</asp:TemplateField>

 <asp:TemplateField ItemStyle-Width = "150px"  HeaderText = "Size">
    <ItemTemplate>
        <asp:Label ID="lblSize" runat="server"
            Text='<%# Eval("Size")%>'></asp:Label>
    </ItemTemplate>
    <EditItemTemplate>
        <asp:TextBox ID="txtSize" runat="server"
            Text='<%# Eval("Size")%>'></asp:TextBox>
    </EditItemTemplate> 
    <FooterTemplate>
        <asp:TextBox ID="txtSize" runat="server"></asp:TextBox>
    </FooterTemplate>
</asp:TemplateField>
           
           
           
        <%--     <asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name" />
            <asp:BoundField DataField="Size" HeaderText="Size" SortExpression="Size" />   --%>
           
           
                
        
            <asp:CommandField HeaderText="Update" ShowEditButton="True" />
                 
                 </Columns>
    </asp:GridView>