Gridview within dynamic dropdownlist null exception

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

<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>

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);
}
}