.keeps showing me an exeption error message that my request data is null ,the return value is a list

Question

This is the service that i use which gets data from firebase

public static async Task<IEnumerable<Specalist>> GetUser()
        {
            List<Specalist> lstUserInfo = (await firebaseClient
   .Child($"Specalists/{PreferencesConfig.Id}").
           OnceAsync<Specalist >()).Select(item => new Specalist 

           {
               name= item.Object.name,
               type= item.Object.type,
               Passsword = item.Object.Passsword,
               ID = item.Object.ID
           }).ToList();
            return lstUserInfo;

        }

And this is the viewmodle :

 private ObservableCollection<Specalist> _specalist = new ObservableCollection<Specalist>();
        private ICommand _deletePatient;

        public ObservableCollection<Specalist> Specalists { get => _specalist; set => SetProperty(ref _specalist, value, nameof(Specalists)); }
Specalists = new ObservableCollection<Specalist>((List<Specalist>)await SpecalistService.GetUser());

Exception occured while processing the request is what the error msg , how can i fix it!