problem is,
Session["islogin"]
.ToString()
your session named "isLogin" is not created so gives null value,
so,
Session["islogin"] = Null
and
in this case run time statement is,
Null.ToString()
and Null are not coverted in string (they haven't this method you are accessing)
so, please make sure that "islogin" session is created and after that you can use it,
else put condition,
if (session["islogin"] != Null)
{
if (Session["islogin"].ToString() == "false")
{
....
}
}
Happy Coding!
:)