To reproduce run the app and make double-click in the area below the last row.

You will get this:

java.lang.NullPointerException
at com.sun.javafx.scene.control.behavior.TreeTableRowBehavior.mouseReleased(TreeTableRowBehavior.java:76)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:79)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:72)


public class EditableTreeTableViewTest extends Application {

    TreeTableView<Person> treeTableView;
    TreeTableColumn<Person, String> firstNameCol;

    @Override
    public void start(Stage primaryStage) {

        initTreeTableView();

        HBox root = new HBox(10.0);
        root.getChildren().add(treeTableView);

        Scene scene = new Scene(root, 400, 300);

        primaryStage.setTitle(VersionInfo.getRuntimeVersion());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

    private void initTreeTableView() {
        treeTableView = new TreeTableView<>();
        treeTableView.setMinWidth(100);
        treeTableView.setPrefWidth(100);
        treeTableView.setMaxWidth(100);
        treeTableView.setColumnResizePolicy(TreeTableView.CONSTRAINED_RESIZE_POLICY);
        
        TreeItem<Person> root = new TreeItem<>(new Person("Anna"));
        root.setExpanded(true);
        treeTableView.setRoot(root);
        treeTableView.showRootProperty().set(true);

        firstNameCol = new TreeTableColumn<>("First name");
        firstNameCol.setCellValueFactory(new Callback<TreeTableColumn.CellDataFeatures<Person, String>, ObservableValue<String>>() {
            @Override
            public ObservableValue<String> call(TreeTableColumn.CellDataFeatures<Person, String> p) {
                return p.getValue().getValue().firstName;
            }
        });
        firstNameCol.setMinWidth(80d);
        
        treeTableView.getColumns().addAll(firstNameCol);

        Person p = new Person("Bob");
        treeTableView.getRoot().getChildren().add(new TreeItem<>(p));

        p = new Person("Cindy");
        treeTableView.getRoot().getChildren().add(new TreeItem<>(p));
        
        p = new Person("Zack");
        treeTableView.getRoot().getChildren().add(new TreeItem<>(p));
    }
    
    class Person {

        public final SimpleStringProperty firstName;

        public Person(String name) {
            this.firstName = new SimpleStringProperty(name);
        }
    }
}