How to turn off messagebox with mouse click?

  private Form Formular()
        {
            Form form1 = new Form();
            form1.Text = "Upozornění";
            return form1;
        }
 protected async override void OnToolMouseDown(MapViewMouseButtonEventArgs e)
        {
            if (e.ChangedButton == MouseButton.Left)
            {
                await QueuedTask.Run(() =>
                {
                    var form1 = Formular();
                    form1.Close();
...
 if (shape == null)
                    {
                        form1 = Formular();
                        form1.ShowDialog();
                        //MessageBox.Show("Nebyla zvolena anotace!");

                        return;
                    }