C# How to check if dialog box is still open?

Question

So I have a background worker that may still be working when the closing event occurs.

I call cancelasync() like so:

void ProgressDialog_Closing(object sender, CancelEventArgs e)
{
  if (_worker.IsBusy)
  {
    //notifies the async thread that a cancellation has been requested.
    _worker.CancelAsync();
  }
}

However, I am getting a crash because of my runworker_completed method, which states that the dialog result cannot be set when the dialog box is not open. How can I check if the dialog box is still open?

void _worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
      if (!Dispatcher.CheckAccess())
      {
        //run on UI thread
        RunWorkerCompletedEventHandler handler = _worker_RunWorkerCompleted;
        Dispatcher.Invoke(handler, null, DispatcherPriority.SystemIdle, new object[] { sender, e });
        return;
      }
      else
      {
        if (e.Error != null)
        {
          error = e.Error;
        }
        else if (!e.Cancelled)
        {
          //assign result if there was neither exception nor cancel
          result = e.Result;
        }
        ProgressBar.Value = ProgressBar.Maximum;
        CancelButton.IsEnabled = false;


        //set the dialog result, which closes the dialog

        if (error == null && !e.Cancelled)
        {
          DialogResult = true;
        }
        else
        {
          DialogResult = false;
        }

      }

    }

Constructor is nothing special, the runworkerthread is what starts the worker and sets the delegate to run.

public bool RunWorkerThread(object argument, Func<object> workHandler)
    {
      //store reference to callback handler and launch worker thread
      workerCallback = workHandler;
      _worker.RunWorkerAsync(argument);


      return ShowDialog() ?? false;
    }

Answer 1

    DialogBox dialogBox = new DialogBox();

    // Show window modally 
    // NOTE: Returns only when window is closed
    Nullable<bool> dialogSelection = dialogBox.ShowDialog();
    if dialogSelection = null // box is still open

http://msdn.microsoft.com/en-us/library/system.windows.window.showdialog(v=vs.110).aspx

I think the dialogSelection is null until the user makes a selection, or the box closes.