Pfthb

I have a login page and and a accounts controller with Login action. When I log in I get redirected to home page(which is good) but after logging in if I re visit the login page it shows the login form again (although I am logged in).
I tried check for session state values but every time I try to use it I get null reference error.

public ActionResult Login(string name, string password, string hash)
 

 if (!string.IsNullOrWhiteSpace(name))
 
 var user = _model.tblUsers.FirstOrDefault(x => x.username == name);
 if (user != null)
 
 if (user.powerLevel == 0)
 
 Session["IsAdmin"] = (user.password == password);
 Session["IsAuthor"] = null;
 Session["IsUser"] = null;
 
 else if (user.powerLevel == 1)
 
 Session["IsAdmin"] = null;
 Session["IsAuthor"] = (user.password == password);
 Session["IsUser"] = null;
 
 else if (user.powerLevel == 2)
 

 Session["IsAdmin"] = null;
 Session["IsAuthor"] = null;
 Session["IsUser"] = (user.password == password);
 
 else
 
 return View("Login");
 
 return RedirectToAction("Index","Posts");
 
 
 return View("Login");
 

so if either of IsAdmin, IsAuthor, IsUser Session is set to true I want to get redirected to homepage. I tried check it with string.IsNullOrWhiteSpace but it doesnt work I always get false even if the Session is set to true