Login page with automatic logout with sessions in ASP.NET

<pre>​


My web.config

 <system.web>
   
	  <authentication mode="Forms">
		  <forms loginUrl="Login.aspx" ></forms>
	  </authentication>
	  <sessionState  timeout="1" ></sessionState>

  </system.web>


Global.asax

protected void Session_Start(object sender,EventArgs e)
        {
            Session["LastActivityTime"] = DateTime.Now;
        }
        protected void Session_End(object sender,EventArgs e)
        {
            DateTime lastActivityTime = (DateTime)Session["LastActivityTime"];
            if (DateTime.Now - lastActivityTime > TimeSpan.FromMinutes(1))
            {
                if (HttpContext.Current != null)
                {
                    FormsAuthentication.SignOut();
                    Response.Redirect("Login.aspx");
                }
            }
        }

My Login.aspx.cs
 

    public partial class Login : System.Web.UI.Page
    {
        ExceptionManager oExceptionManager = new ExceptionManager();
        MyProjectBLL oProjectBll = new MyProjectBLL();
        
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Session["LastActivityTime"] != null)
            {
                Session["LastActivityTime"] = DateTime.Now;
            }
        }

        protected void btnSubmit_Click(object sender, EventArgs e)
        {
            DataTable dt = new DataTable();          
            try
            {
                dt = oProjectBll.GetCredentials();
                if (dt.Rows.Count > 0)
                {
                    foreach (DataRow dataRow in dt.Rows)
                    {
                        string storedUsername = dataRow["USERNAME"].ToString();
                        string storedPassword = dataRow["PASSWORD"].ToString();
                        if (storedUsername == txtName.Text && storedPassword == txtPwd.Text)
                        {                           
                            Response.Redirect("~/CommonControls/MasterPageWebForm.aspx");
                        }
                    }
                }
            }
            catch(Exception ex)
            {
                oExceptionManager.LogError(ex.Message, "Inside btnSubmit_Click function from Login.aspx.cs");
            }
        }
    }



​

i was trying to do automatic logout for my web application if the user is idle or not visiting the page it will automatically redirect to login page what i have tried is above code 

im getting Object reference not set to an instance of an object at   FormsAuthentication.SignOut(); pls help me to solve it