"Object reference not set to an instance of an object" - getting this error while binding dropdown using jquery

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C#

ASP.NET

Ajax

jQuery

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Hello All,

I am using jquery ajax to populate a whole form on selection of an item. The form contains 4-5 dropdown lists and 3 checkbox list and few textboxes. So, I planned to bind the values using c# from code behind instead of writing fucntions in jquery itself for each dropdowns /

The scenario has been explained as below with the help of a sample:

testForm.aspx page

function shw(id) {
            alert(id);

            $.ajax({
                type: "post",
                url: "testForm.aspx/popu",
                data: '{"uid" : ' + id + '}',
                //dataType: "json",
                contentType: "application/json; charset=utf-8",
                success: function (resp) {
                    alert(resp.d);
                },
                error: function (xhr, errorType, exception) {
                    alert(errorType + " | " + exception);
                }
            });
        }

<asp:ListBox ID="ListBox1" runat="server" onchange="shw(this.value);">
            <asp:ListItem Text="A" Value="1"></asp:ListItem>
            <asp:ListItem Text="B" Value="2"></asp:ListItem>
            <asp:ListItem Text="C" Value="3"></asp:ListItem>
            <asp:ListItem Text="D" Value="4"></asp:ListItem>
            <asp:ListItem Text="E" Value="5"></asp:ListItem>
        </asp:ListBox>

testForm.aspx.cs page

[WebMethod]
        public static void popu(string uid)
        {
            //JavaScriptSerializer js = new JavaScriptSerializer();
            //return js.Serialize("Value = " + ID);

            //return "Value = " + uid;

            using (testForm obj = new testForm())
            {
                obj.bindDrop1();
            }
        }

private void bindDrop1()
        {
            DataTable tbl = new DataTable();
            tbl.Columns.Add("qid", typeof(Int32));
            tbl.Columns.Add("desc");

            DataRow dr;

            for (int i = 1; i <= 5; i++)
            {
                dr = tbl.NewRow();
                dr["qid"] = i;
                dr["desc"] = "Q" + i;

                tbl.Rows.Add(dr);
                tbl.AcceptChanges();
            }

            ddl1.DataTextField = "desc";
            ddl1.DataValueField = "qid";
            ddl1.DataSource = tbl;
            ddl1.DataBind();
        }

The above bold line in bindDrop1() method throws the error:
"Object reference not set to an instance of an object."
When I looked for "ddl1" object, its showing NULL.
Well, I can understand that the object (ddl1) is NULL and so this error is. But, I cannot be able to understand why and how?
Because, the "page_load" event is called for first time and that time only all the controls like ddl1, ddl2 etc. are getting created. So, why its showing NULL?