未将对象引用设置到对象的实例

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data;

public partial class _13_11 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        DataSet ds = new DataSet();
        //创建datatable对像
        DataTable dt = new DataTable("t_aritcle");
        //创建列
        DataColumn dtc=new DataColumn ("文章id",typeof(Int32));
        dt.Columns.Add(dtc);
        dtc = new DataColumn("标题", typeof(string));
        dt.Columns.Add(dtc);
        dtc = new DataColumn("作者", typeof(string));
        dt.Columns.Add(dtc);

        // 添加数据到datatable中
        DataRow dtr = dt.NewRow();
        dtr["文章id"]=1;
        dtr["标题"]="天降冰雹";
        dtr["作者"]="王二";
        dt.Rows .Add (dtr);

        dtr=dt.NewRow ();
        dtr ["文章id"]=2;
        dtr ["标题"]="中大奖了";
        dtr["作者"] = "幽忧";
        
        //添加到dataset中
        ds.Tables .Add (dt);
        //绑定到gridview
        GridView1.DataSource=ds.Tables ["t_article"].DefaultView;
        GridView1.DataBind();
    }
   
}
  GridView1.DataSource=ds.Tables ["t_article"].DefaultView;
这名出现：未将对象引用设置到对象的实例，请各位帮我查看一下，谢了。