Object refrence not set error List

   public class RootObject
        {
            public string UserEmail { get; set; }
            public List<string> UserRoles { get; set; }
        }

My class and i am using in this

 public List<RootObject> GetUser()
        {
            List<RootObject> rbl = new List<RootObject>();
            RootObject rb = new RootObject();
            var manager = new UserManager();
            ApplicationDbContext db = new ApplicationDbContext();
            var list = db.Users.ToList();
            foreach (var users in list)
            {

                ApplicationUser user = db.Users.Where(u => u.Id.Equals(users.Id, StringComparison.CurrentCultureIgnoreCase)).FirstOrDefault();
                rb.UserEmail = user.Email;
                var roles = manager.GetRoles(user.Id);
                 foreach (var role in roles)
                {
                    rb.UserRoles.Add(String.Join(",", role));
                }
                rbl.Add(rb);
            }
            return rbl.ToList();
        }

I dont know how to initialize Error is because of this

public List<string> UserRoles