How to show data in gridview asynchronously in asp.net?

Question

My code is properly work in desktop application show data asncrounsly in data gridview but when I use same code in web form it throws exception GridView1 was null. at GridView1.DataSource = table;.While debugging data is completely working just want to put that data into gridview.What changes should i made to acheive my task.

    public partial class _Default : System.Web.UI.Page
    {
        public class NameAndScore
        {
            public string Name { get; set; }
            public string Score { get; set; }
        }
     public _Default()
     {

        InitTable();
     }
        DataTable table;
        HtmlWeb web = new HtmlWeb();

        private async Task<List<NameAndScore>> GameRankingsFromPage(int pagenum)
        {
            string url = "https://www.ebay.com/sch/i.html?_nkw=xbox+one&_in_kw=1&_ex_kw=&_sacat=0&LH_Complete=1&_udlo=&_udhi=&_ftrt=901&_ftrv=1&_sabdlo=&_sabdhi=&_samilow=&_samihi=&_sadis=15&_stpos=&_sargn=-1%26saslc%3D1&_salic=1&_sop=12&_dmd=1&_ipg=50&_fosrp=1";
            //    string url = "https://www.gamerankings.com/browse.html";

            if (pagenum != 0)
                url = "https://www.ebay.com/sch/i.html?_sacat=0&LH_Complete=1&_udlo=&_udhi=&_ftrt=901&_ftrv=1&_sabdlo=&_sabdhi=&_samilow=&_samihi=&_sadis=15&_stpos=&_sop=12&_dmd=1&_fosrp=1&_nkw=xbox+one&_pgn=" + pagenum.ToString() + "&_skc=50&rt=nc";

            var doc = await Task.Factory.StartNew(() => web.Load(url));

            var namenodes = doc.DocumentNode.SelectNodes("//*[contains(@id,'item')]/h3/a");
            var scorenodes = doc.DocumentNode.SelectNodes("//*[contains(@id,'item')]/ul[1]/li[1]/span");
            var names = namenodes.Select(node => node.InnerText.Trim('\r', '\n', '\t'));
            var scores = scorenodes.Select(node => node.InnerText.Trim('\r', '\n', '\t'));


            if (namenodes == null || scorenodes == null)
                return new List<NameAndScore>();

            return names.Zip(scores, (name, score) => new NameAndScore() { Name = name.ToString(), Score = score.ToString() }).ToList();
        }
        protected async void Page_Load(object sender, EventArgs e)
        {
            int pageNume = 0;
            int id = 0;
            var rankings = await GameRankingsFromPage(0);
            while (rankings.Count > 0)
            {
                foreach (var ranking in rankings)
                     table.Rows.Add(id++, ranking.Name, ranking.Score);
                GridView1.DataSource = table;
                GridView1.DataBind();
                pageNume++;
                rankings = await GameRankingsFromPage(pageNume);
            }

        }
        private void InitTable()
        {
            table = new DataTable("Xbox Prices");
            table.Columns.Add("ID", typeof(string));
            table.Columns.Add("Name", typeof(string));
            table.Columns.Add("Score", typeof(string));
            GridView1.DataSource = table;
            GridView1.DataBind();

        }
    }