You should not use .ToString() over there. Instead of that you can use:
if (!string.IsNullOrEmpty(Request.UrlReferrer))
Or you could have use
Convert.ToString(Request.UrlReferrer)
It handles null value and that is the main difference between ToString() and Convert.ToString().
Use the below code to resolve your purpose:
if (!IsPostBack)
{
var MasterPage = Master;
var a = Request.UrlReferrer;
string url = HttpContext.Current.Request.Url.AbsoluteUri;
if (a != null && url == "your Required url")
{
if (MasterPage != null)
{
MasterPage.FindControl("Admin_Header1").Visible = false;
MasterPage.FindControl("tdLeft").Visible = false;
MasterPage.FindControl("footertable").Visible = false;
}
}
}