/// <summary>
        /// Checks null user or no user and adds it.
        /// </summary>
        /// <param name="user">User.</param>
        /// <returns>True if there is no user with the same input or null user.</returns>
        public static bool AddUser(User user)
        {
            if (user != null)
            {
                foreach (var item in users)
                {
                    if (user.Email == item.Email)
                        return false;
                }

                users.Add(user);
                logedInUser = user;
                UpdateDatabase();
                return true;
            }

            return false;
        }
 /// <summary>
 /// Adds user.
 /// </summary>
 /// <param name="user">User.</param>
 /// <returns>True if there is no user with the same input.</returns>
 public static bool AddUser(User user)
 {
     return UsersDatabase.AddUser(user);
 }
        /// <summary>
        /// User login.
        /// </summary>
        /// <param name="email">Email.</param>
        /// <param name="password">Password.</param>
        /// <returns>True if there is existing user.</returns>
        public static bool LoginUser(string email, string password)
        {
            foreach (var user in users)
            {
                if (user != null)
                {
                    if (user.Email == email && user.Password == password)
                    {
                        logedInUser = user;
                        UpdateDatabase();
                        return true;
                    }
                }
            }

            return false;
        }