Why this problem occurs?

Hi, I have a droid project and a core project in net standard 2.0. I've using firebase for the backend and MVVM cross to comunicate the parts.

I create a service with basic crud operations using generics:

     public async Task<IReadOnlyCollection<FirebaseObject<T>>> GetAll <T(string endpoint)
               where T: class
        {
            var token = _storageService.GetToken();

                var res = await this._firebaseClient
                 .Child(endpoint)
                 .WithAuth(token.AccessToken)
                 .OnceAsync<T>();

                return res;
        }

The idea is call this method from different view models but doesn't works, I've implemented in this way:

In ctor of viewmodel:

        public TasksViewModel(        
                IMvxNavigationService navigationService,
                IBasicCRUDService crudService)
            {
                this.navigationService = navigationService;
                this._crudService = crudService;

                this.LoadTasksCommand = new MvxAsyncCommand(this.LoadTasks);
            }

And the method:

            public IMvxAsyncCommand LoadTasksCommand { get; private set; }
            public async System.Threading.Tasks.Task LoadTasks()
            {
                var res = await _crudService.GetAll<Models.Task>("tasks");
            }

But when the call to firebase in BasicCrudService occurrs, the app stop working and throws this error:

    (23746): /Users/builder/jenkins/workspace/xamarin-android-d15-7/xamarin-android/external/mono/mono/mini/debugger-agent.c:4923: Could not execute the method because the containing type is not fully instantiated. assembly: type: member:(null) signature: <none>
    (23746): Fatal signal 6 (SIGABRT), code -6 in tid >23746 (ame.Doing_Droid)

Any idea?.

Thanks to all and sorry for mi bad English!