gridview with dynamic dropdownlist control

Question

I have a GridView with template fields and one of the template field is a dropdownlist which is dynamically added into the gridview control. It's working properly on page load when I click the button control to the get the values from the template fields it will throw a null reference exception. Please correct my error, I am in pathetic situation. I attached the source code.

Regards

DESIGN

    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false"
        OnRowCreated="GridView1_RowCreated">
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:Label ID="id" runat="server" Text='<%# Eval("RGNO") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:Label ID="name" runat="server" Text='<%# Eval("NAME") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:Label ID="lastname" runat="server" Text='<%# Eval("DOB") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>

CODE BEHIND

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
            BindGridView();
    }

    protected void GridView1_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            DropDownList ddlResult = new DropDownList();
            ddlResult.ID = "ddlResult";
            ddlResult.Items.Insert(0, new ListItem("PASS", "0"));
            ddlResult.Items.Insert(1, new ListItem("FAIL", "1"));

            PlaceHolder p = (PlaceHolder)e.Row.FindControl("PlaceHolder1");
            p.Controls.Add(ddlResult);
        }
    }

    private void BindGridView()
    {
        GridView1.DataSource = CreateDataSource();
        GridView1.DataBind();
    }

    public DataTable CreateDataSource()
    {
        DataTable dt = new DataTable();
        DataRow dr;

        dt.Columns.Add(new DataColumn("RGNO", typeof(string)));
        dt.Columns.Add(new DataColumn("NAME", typeof(string)));
        dt.Columns.Add(new DataColumn("DOB", typeof(string)));

        dr = dt.NewRow();
        dr["RGNO"] = "1001";
        dr["NAME"] = "PRAKASH";
        dr["DOB"] = "10-10-2003";
        dt.Rows.Add(dr);
        return dt;
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        foreach (GridViewRow row in GridView1.Rows)
        {
            DropDownList dd1 = (DropDownList)row.FindControl("ddlResult");
            Response.Write(dd1.SelectedItem.Text);
        }
    }