JavaScript 2 Java Bridge seems to be broken; browser objects are unavailable to scripts.
The easiest way to reproduce is to run the following code:

public class Main extends Application {

    WebEngine e;
    @Override
    public void start(Stage stage) throws Exception {
        WebView wv = new WebView();
        e = wv.getEngine();
        Scene scene = new Scene(wv);

        JSObject o = (JSObject) e.executeScript("window;");
        System.out.println(o);
        
        stage.setScene(scene);
        stage.show();
    }
}

The same code and tests seem to be working fine on Windows.