private void Button_Click(object sender, RoutedEventArgs e)
 {
     if (GradesData.TestGradesEmpty() == true)
     {
         MessageBox.Show("True");
     }
     else
     {
         MessageBox.Show("False");
     }
 }
        public MainWindow()
        {
            InitializeComponent();
            FillStudStatusChoices();

            if (GradesData.TestGradesEmpty() == true)
            {
                GradesData.CopyTestGrades();
            }


            if (StudentData.TestStudentsIfEmpty() == true)
            {
                StudentData.CopyTestStudents();
            }
        }