在ViewModel中使用Observable Collection时，出现Null引用异常

Question

我创建了一个Observable集合，目的是从数据库中检索记录以便在DataGrid控件中查看。运行应用程序时，在以下代码部分中收到Null Reference异常：

public static async Task<MainViewModel> CreateAsync()
    {
        var viewModel = new MainViewModel();
        viewModel.RecordingDevices = new ObservableCollection<DeviceInformation>();

        var outputDevices = await DeviceInformation.FindAllAsync(DeviceClass.AudioRender);
        foreach (var device in outputDevices.Where(d => d.IsEnabled))
        {
            viewModel.OutputDevices.Add(device);
        }

        //SelectedOutputDevice = OutputDevices.FirstOrDefault(d => d.IsDefault);
        var outputId = MediaDevice.GetDefaultAudioRenderId(AudioDeviceRole.Default);
        viewModel.SelectedOutputDevice = viewModel.OutputDevices.FirstOrDefault(d => d.Id == outputId);

        var inputDevices = await DeviceInformation.FindAllAsync(DeviceClass.AudioCapture);
        foreach (var device in inputDevices.Where(d => d.IsEnabled))
        {
            viewModel.RecordingDevices.Add(device);
        }

        var defaultId = MediaDevice.GetDefaultAudioCaptureId(AudioDeviceRole.Default);
        viewModel.SelectedInputDevice = viewModel.RecordingDevices
            .FirstOrDefault(d => d.Id == defaultId);

        viewModel.ActivityLogEntries = new ObservableCollection<ActivityLogEntry>();
        try
        {
            if (viewModel.ActivityLogEntries != null)
            {
                viewModel.ActivityLogEntries = await ActivityLogService.GetActivityLogEntriesAsync();
            }
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine("Activity log error ("+ex.Message+"Details: "+ex.InnerException);
        }

        return viewModel;
    }

如何解决这个空引用，因为它与ActivityLogEntries可观察的集合有关？