If i try to run a JavaFX project with more than 2 scenes it crushes

With only 2 scenes my project runs fine.If i add more,it crushes and the following error pops-up:Exception in Application start method
I am not using fxml and i can’t find a way to make it run
Here is a sample of my code,thanks in advance
Imports are fine and Scenes are created separately

public class CarRentalServiceFX extends Application {

    static Stage mainStage;
    static Scene mainScene, cityScene, storeScene, clientScene, vehicleScene, rentalScene;
	public void start(Stage primaryStage)  {

        mainStage = primaryStage;
		SceneCreator mainSceneCreator = new MainSceneCreator(650, 300);
		SceneCreator storeSceneCreator = new StoreSceneCreator(650, 300);
		SceneCreator citySceneCreator = new CitySceneCreator (650, 300);
		SceneCreator clientSceneCreator = new ClientSceneCreator (650, 300);
		SceneCreator vehicleSceneCreator = new VehicleSceneCreator (650, 300);
		SceneCreator rentalSceneCreator = new RentalSceneCreator (650, 300);
		mainScene = mainSceneCreator.createScene();
		storeScene = storeSceneCreator.createScene();
		cityScene = citySceneCreator.createScene();
		clientScene = clientSceneCreator.createScene();
		vehicleScene = vehicleSceneCreator.createScene();
		rentalScene = rentalSceneCreator.createScene();

        primaryStage.setTitle("CarRentalServiceFX Window");


	public static void main(String[] args) {