C# - Check For Session Before Returning?

I have a function inside a utility class which returns the current session User ID.
It throws object reference not set to instance of object ? How do I check it for null & remove this error ?

public static string GetSessionUserID
{
get
{
string userID = "";
if (System.Web.HttpContext.Current.Session["userID"].ToString() != null)
{
userID = System.Web.HttpContext.Current.Session["userID"].ToString();
}
if (userID == null)
{
throw new ApplicationException("UserID is null");
}
else
return userID;
}
}