Error idk how to fix

public class AppData : ObservableObject
    {
        private string accountToken;
        private string userName;
        private string userImageSource;
        private string userId;

        public string AccountToken { get => accountToken; set { accountToken = value; OnPropertyChanged(); } }

        public string UserName { get => userName; set { userName = value; OnPropertyChanged(); } }

        public string UserImageSource { get => userImageSource; set { userImageSource = value; OnPropertyChanged(); } }

        public string UserId { get => userId; set { userId = value; OnPropertyChanged(); } }
    }

class ObservableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string name = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }