private void DeleteSubject_Click(object sender, RoutedEventArgs e)
        {
            var dg = sender as DataGrid;

            for (var vis = sender as Visual; vis != null; vis = VisualTreeHelper.GetParent(vis) as Visual)
            {
                if (vis is DataGridRow)
                {
                    if (((System.Windows.FrameworkElement)vis).DataContext is DTOSubject)
                    {
                        var subjectId = ((DTO.DTOSubject)((System.Windows.FrameworkElement)vis).DataContext).SubjectId;
                        if (subjectId != -1)
                        {
                            client.DeleteSubject(subjectId);
                            SetListOfSubjects();
                        }
                    }
                }
            }
        }