Mahhaps - Show Dialog from library class

Question

I use IDialogCoordinator on ViewModel and everything works fine.

But if in viewmodel i have instance of other class, it doesn't work. I try to pass dialogCoordinator in constructor, but I've got a message "Context is not registered".

How to achieve that?

Viewmodel:

public viewModel
{
  private IDialogCoordinator dialogCoordinator;
  public ICommand SyncCommand { get { return new RelayCommand(SyncCommand_ExecutedAsync); } }  

  public viewModel(IDialogCoordinator _coordinator)
  {
    coordinator = _coordinator;
  }

  private void SyncCommand_ExecutedAsync()
  {
        var sync = new IndexUpdater(dialogCoordinator);
        var b = sync.Start().Result;
  }
}

IndexUpdater:

public IndexUpdater(IDialogCoordinator _dialogCoordinator)
    {
        dialogCoordinator = _dialogCoordinator;
    }

public async Task<bool> Start(bool reset = false)
    {
        var output = true;

        ProgressDialogController controller = await dialogCoordinator.ShowProgressAsync(this, "HEADER", "MESSAGE", true, new MetroDialogSettings { NegativeButtonText = "Close" });
         controller.Minimum = 0;
        controller.Maximum = 100;

        for (int x = 0; x < 100; x++)
        {
            if (controller.IsCanceled)
            {
                output = false;
                break;
            }

            Thread.Sleep(200);

            controller.SetProgress(x + 1);
            controller.SetMessage("x = " + x);
        }

        await controller.CloseAsync();
        return output;
}

In this case I got Exception: "InvalidOperationException: Context is not registered. Consider using DialogParticipation.Register in XAML to bind in the DataContext."

If I put ProgressDialogController directly in viewModel.SyncCommand_ExecutedAsync() method, then it works.

How to Register IndexUpdater class or how to pass dialogCoordinator there?

Thanks in Advance