JavaFX - Loading/creating multiple controllers

Question

I have researched this for a couple hours now, and even though there is a lot about "multiple controllers", none of the solutions worked for my problem. I don't know if I am just structuring my program wrong, so I need some input.

I got a "MainView" (App class with AppView and AppController) and a "SubView" (SubView and SubViewController). In App, I am loading the mainView as primaryStage and loading the AppController. I created a stageManager class to handle different views. My problem is: Where and how do I create/load my subViewController?

public class App extends Application {

    private Stage primaryStage;
    private AnchorPane rootLayout;
    private AppController appController;

    MainModel model = new MainModel();
    StageManager stageManager;

    @Override
    public void start(Stage primaryStage) {
        this.primaryStage = primaryStage;
        this.primaryStage.setTitle("App");

        initRootLayout();
        appController.initModel(model);
        appController.setStageManager(stageManager);
    }

    public void initRootLayout() {
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(getClass().getResource("mainView.fxml"));
            rootLayout = (AnchorPane) loader.load();
            appController = loader.<AppController>getController();

            Scene scene = new Scene(rootLayout);
            primaryStage.setScene(scene);
            primaryStage.show();

            stageManager = new StageManager(primaryStage, rootLayout, appController);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

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




public class AppController implements Initializable {

    private MainModel model;
    private StageManager stageManager;

    @FXML
    private Button btn;

    @FXML
    private void handleButtonAction(ActionEvent event) {
        stageManager.changeScene("subView.fxml");
    }

    @Override
    public void initialize(URL url, ResourceBundle rb) {
    }   

    public void initModel(MainModel model) {
        this.model = model;
    }

    public void setStageManager(StageManager stageManager){
        this.stageManager = stageManager;
    }
}


public class StageManager {

    private Stage primaryStage;
    private AnchorPane rootLayout;
    private AppController appController;

    public StageManager(Stage primaryStage, AnchorPane rootLayout, AppController appController){
        this.primaryStage = primaryStage;
        this.rootLayout = rootLayout;
        this.appController = appController;
    }

    public void changeScene(String scene){
        try {
            Parent parentPane = FXMLLoader.load(getClass().getResource(scene));
            primaryStage.getScene().setRoot(parentPane);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


public class SubController implements Initializable {

    private MainModel model;

    @Override
    public void initialize(URL url, ResourceBundle rb) {
    }    

    public void setModel(MainModel model){
        this.model = model;
    }    
}


<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.Pane?>
<?import javafx.scene.text.Font?>

<AnchorPane id="AnchorPane" prefHeight="524.0" prefWidth="850.0" xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="app.SubController">
   <children>
      <Label fx:id="lbl" layoutX="52.0" layoutY="13.0" prefHeight="38.0" prefWidth="214.0" text="just a label">
         <font>
            <Font name="Consolas" size="24.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

I tried the following things to load the other controller: First I tried doing it the same way I am loading the appController (since that works), but without setting the view as the scene. So in the initRootLayout() method I simply added:

FXMLLoader subloader = new FXMLLoader();
subloader.setLocation(getClass().getResource("subView.fxml"));
subController = loader.<SubController>getController();

This didn't work. In fact something really weird happened. When I tried calling a method from subController, I got a NullPointerException. Debugging revealed that the subController is being loaded, but right after its loaded it disappears and subController Object is just "null". Why is that?

The next thing I tried after reading about this approach was to add the subController as a FXML variable. In the AppController I added @FXML private SubController subController; and in the AppController's initialize() method I tried making a call from the controller:

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        subController.setModel(model);
    } 

In this case, I am getting a NullPointerException, but it is actually pointing to the line where I call appController.initModel(model); in the App class. Even though this line works fine when I am not trying to use the subController. So, I am extremely confused and I think I need a whole new structure for this. Any kinda help would be greatly appreciated! :)

EDIT: This seems to be working: I changed the changeScenes() method to this:

public void changeScene(String scene){
    try {
     FXMLLoader loader = new FXMLLoader(getClass().getResource(scene));
     Parent root = (Parent) loader.load();
     subViewController = loader.getController();
     subViewController.setModel(mainModel);
     Scene newScene = new Scene(root);
     Stage newStage = new Stage();
     newStage.setScene(newScene);
     newStage.show();
  } catch (Exception e) {
     e.printStackTrace();
  }
}