Populating Gridview's Drop Down List depending on database value

Question

I would like to get some help here in my code, what am I trying to do is to populate my drop down list which shows the available quantity (stored in database) but when I try to find the control in the row I get a null reference exception it says row was null.

Here is the template field that has my drop down list

<asp:TemplateField HeaderText="Quantity" SortExpression="Quantity">
   <EditItemTemplate>
        <asp:DropDownList ID="ddlQuantity" runat="server">
        </asp:DropDownList>
    </EditItemTemplate>
    <ItemTemplate>
        <asp:Label ID="Label4" runat="server" Text='<%# Bind("Quantity") %>'></asp:Label>
    </ItemTemplate>
</asp:TemplateField>

and my code behind

protected void Cart_GV_RowEditing(object sender, GridViewEditEventArgs e) {
        Cart_GV.EditIndex = e.NewEditIndex;
        GridViewRow row = (sender as Control).Parent.Parent as GridViewRow;
        DropDownList ddlQuantity = row.FindControl("ddlQuantity") as DropDownList;
        string cartitemid = Cart_GV.DataKeys[e.NewEditIndex].Value.ToString();
        conn.Open();
        SqlCommand checkQuantity = new SqlCommand("SELECT OnHandQTY FROM Inventory INNER JOIN Cart_Item ON Inventory.ProductID = Cart_Item.ProductID WHERE CartItemID = '" + cartitemid + "';", conn);
        int availalbleQuantity = (int)checkQuantity.ExecuteScalar();
        conn.Close();
        for (int i = 1; i <= availalbleQuantity; i++) {
            ListItem item = new ListItem(i.ToString(), i.ToString());
            ddlQuantity.Items.Add(item);
        }
        ViewUserCart();
    }

Answer 1

Please Try this one

protected void Cart_GV_RowEditing(object sender, GridViewEditEventArgs e) {
    Cart_GV.EditIndex = e.NewEditIndex;
    GridViewRow row = (sender as Control).Parent.Parent as GridViewRow;
    DropDownList ddlQuantity = row.FindControl("ddlQuantity") as DropDownList;
    string cartitemid = Cart_GV.DataKeys[e.NewEditIndex].Value.ToString();
    conn.Open();
    SqlCommand cmd = new SqlCommand("SELECT OnHandQTY FROM Inventory INNER JOIN Cart_Item ON Inventory.ProductID = Cart_Item.ProductID WHERE CartItemID = '" + cartitemid + "';", conn); 
    SqlDataAdapter sda = new SqlDataAdapter(cmd);  
    DataTable dt = new DataTable();  
    sda.Fill(dt);  
    con.Close();  
    ddlQuantity.DataSource = dt;  
         
    ddlQuantity.DataTextField = "OnHandQTY";  
    ddlQuantity.DataValueField = "OnHandQTY";  
    ddlQuantity.DataBind();
    ViewUserCart();
}