To reproduce:

- run
- click header
- expected: data sorted
- actual: NPE thrown

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.scene.control.TableView.sort(TableView.java:1508)
at javafx.scene.control.TableView.doSort(TableView.java:1578)
at javafx.scene.control.TableView.lambda$new$37(TableView.java:532)
at javafx.scene.control.TableView$$Lambda$86/19352785.onChanged(Unknown Source)

Similar to RT-37674, the options to fix are
- either allow null as value and make sure that each and every internal access is guarded against null
- or disallow and clearly document that it is not a valid value (and cleanup internals and remove all guards)

/**
 * Issue TableView throws NPE on sorting column if selectionModel property value is null.
 *
 * @author Jeanette Winzenburg, Berlin
 */
public class TableSortNPEIfNullSelectionModel extends Application {
    private final ObservableList<Locale> data =
            FXCollections.observableArrayList(Locale.getAvailableLocales()
                    );

    private Parent getContent() {
        // instantiate the table with null items
        TableView<Locale> view = new TableView<Locale>(data);
        TableColumn<Locale, String> column = new TableColumn<>(
                "Language");
        column.setCellValueFactory(new PropertyValueFactory<>("displayLanguage"));
        // either click on header for sorting
        view.getColumns().addAll(column);
        view.setSelectionModel(null);
        // or add column to sort order immediately
        //view.getSortOrder().add(column);
        Pane parent = new HBox(100);
        parent.getChildren().addAll(view);
        parent.setPadding(new Insets(20));
        return parent;
    }
    
    @Override
    public void start(Stage primaryStage) throws Exception {
        Scene scene = new Scene(getContent());
        primaryStage.setTitle(System.getProperty("java.version"));
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch();
    }
    
}