The following code does not work after recent changes:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.stage.Stage;

public class Test extends Application {
    public static void main(String[] args) {
        Application.launch(args);
    }

    public void start(Stage stage) {
        TreeItem<String> root = new TreeItem<String>();
        add(root, "first"); // works

        TreeView<String> view = new TreeView<String>(root);
        view.setShowRoot(false);
        add(root, "second"); // fails

        stage.setScene(new Scene(view));
        stage.setVisible(true);
    }

    private static void add(TreeItem<String> parent, String child) {
        parent.getChildren().add(new TreeItem<String>(child));
    }
}