Session clear some time or not set

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ASP.NET

i stored session on dropdownchange(DropDownList1_SelectedIndexChanged) event in asp.net.

and submit time i use this session but on submit time it's throwing error instance not set .

i check many time it's work mostly perfectly . ex. if i run project and out of 10 time it's work 9 time perfectly and 1 time it's instance not set.

can i use hidden variable in instead of session . and what is hidden variable life ?

What I have tried:

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
 var announcementData = JsonConvert.DeserializeObject<AnnouncementDataList>(json.ToString());
                Session["announcementData"] = null;
                Session["announcementData"] = announcementData;
        }


//on sendnow i requried session but clear some time or not set 

  protected void SendNow_Click(object sender, EventArgs e)
        {
 var selectedNews = apiresult.Items[apiresult.SelectedIndex].Text;
                    NewsDataList newsData = new NewsDataList();
                    if ((NewsDataList)Session["newsApiResponse"] != null)
                    {
                        newsData = (NewsDataList)Session["newsApiResponse"];
                        noti.newsdata = newsData.data.Where(x => x.Headline == selectedNews).FirstOrDefault();
                    }
}