private void BtnEdit_Click(object sender, RoutedEventArgs e)
        {
            if (ViewModelLocator.ScoresListViewModel.SelectedScore != null)
            {
                var window = new AddScoreWindow();
                window.Owner = this;
                window.WindowStartupLocation = WindowStartupLocation.CenterOwner;

                SCORE changedScore = ViewModelLocator.ScoresListViewModel.SelectedScore as SCORE;
                window.DataContext = changedScore;

                var result = window.ShowDialog();

                if (result == true)
                {
                    ViewModelLocator.ScoresListViewModel.EditScore(changedScore);
                    ViewModelLocator.ScoresListViewModel.ComputeGrade();
                    ViewModelLocator.SelectedSubjectViewModel.SelectedActivity.Grade = ViewModelLocator.ScoresListViewModel.ComputeConvertedGrade();
                }
            }
        }
        private void BtnAddActivity_Click(object sender, RoutedEventArgs e)
        {
            var window = new AddScoreWindow();

            window.Owner = this;
            window.WindowStartupLocation = WindowStartupLocation.CenterOwner;

            SCORE newScore = new SCORE();

            newScore.Date      = DateTime.Now;
            window.DataContext = newScore;

            var result = window.ShowDialog();

            if (result == true)
            {
                ViewModelLocator.ScoresListViewModel.AddNewScore(newScore);
                ViewModelLocator.ScoresListViewModel.ComputeGrade();
                ViewModelLocator.SelectedSubjectViewModel.SelectedActivity.Grade = ViewModelLocator.ScoresListViewModel.ComputeConvertedGrade();
            }
        }