NullReferenceException in ASP.NET

Your FindControl method causes this error. You are attempting to access your DropDownList that is located inside a GridView  in which your GridView is not bounded with data yet. You need to move your code for populating your DropDownList at RowDataBound event to avoid null reference error.

Here's a quick example:

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e) {
            if (e.Row.RowType == DataControlRowType.DataRow) {
                DropDownList ddl1 = (DropDownList)e.Row.FindControl("DropDownList1");
                //populate your dropdownlist here
            }
}