private void AddClassificationButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                string conceptName;
                int    conceptId;

                if (ConceptsRootComboBox.SelectedValue != null)
                {
                    conceptName = ConceptsRootComboBox.SelectedValue.ToString();
                    conceptId   = _concepts.First(c => c.Value == conceptName).Key;
                }
                else if (ConceptsRootComboBox.Text.Length != 0)
                {
                    conceptName = ConceptsRootComboBox.Text;
                    conceptId   = _SQLClient.InsertConceptIdentity(conceptName);
                }
                else
                {
                    throw new ArgumentException("Поле с понятием должно быть заполнено.");
                }

                _SQLClient.InsertClassification(
                    ClassificationTypeComboBox.Text,
                    conceptId,
                    ClassificationBaseTextBox.Text
                    );
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Error");
            }

            Frames.Classifications.Instance.SelectClassifications();
            Frames.Concepts.Instance.SelectClassifications();
            Graphs.TreeVisualizationPage.Instance.SelectClassifications();
        }