Error When Call IMvxCommand in Unit Testing

Kode unit testing di bawah ini ketika memanggil SampleClickCommand menyebabkan error Null Object Reference.

[TestMethod]
       public void WhenPlayClick_SampleMusicIsPlaying()
       {
           _mockAudioService.Setup(arg => arg.PlayMusic(It.IsAny<string>()));
           _mockAudioService.Setup(arg => arg.IsMusicPlaying).Returns(true);
           _scoreLibraryCellViewModel = new ScoreLibraryCellViewModel(_mockAudioService.Object, _mockFileAccessService.Object, _mockDialogService.Object);
           _scoreLibraryCellViewModel.SampleClickCommand.Execute();
 
 
 
       }

Untuk meresolve error ini register type untuk IMvxStringToTypeParser saat initialisasi.

var ioc = MvxSimpleIoCContainer.Initialize();
          ioc.RegisterSingleton<IMvxStringToTypeParser>(new MvxStringToTypeParser());