javafx IllegalArgumentException (is already set as root of another scene)

Question

I have problem with changing scenes in my application which looks like

Main screen > Login screen

I am storing screens in main file as <

Answer 1

The exception is pretty self-explanatory: the anchor pane cannot be the root of two different scenes. Instead of creating a new scene every time, just replace the root of the existing scene:

@FXML
private void LogIn(ActionEvent event) {
    Button button = (Button) event.getSource();
    Scene scene = button.getScene();
    Parent root = MyApplication.pages.get("LoginPage");
    scene.setRoot(root);
}