Java GUI (using FXML):

I am creating an application where I'm combining FXML and regular javaFX to create an application. I'm, however, experiencing problems writing EventHandlers for a Stage-subclass called AddItemWindow that generates custom windows.

The application shows a list of items (keys and weapons) in an inventory. The user can add items, and needs to press a button to add the item of his choice (Add Key or Add Weapon).

A new window then pops up, where the user has to input the relevant data. It will generate a GridPane with the TextFields where the user can input the data. This will be a custom GridPane, depending on the ItemType. It will then load the FXML and add the GridPane.

With the below code, I am getting LoadExceptions for my SetOnAction-code for the buttons cancelling the window or confirming the new item.

Would any-one know where I'm making an error?

/* StartUp Class*/ package main;

//imports from javafx and java
import domain.DomainController;
import gui.OverviewWindow;

public class StartUpGUI extends Application {

    @Override
    public void start(Stage primaryStage) {
        Parent root = new OverviewWindow(new DomainController());

        Scene scene = new Scene(root);

        primaryStage.setTitle("Weapons and Keys");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String... args) {
        Application.launch(StartUpGUI.class, args);
    }

}

/* OverviewWindow, subclass of BorderPane */ package gui;

//imports from javafx and java
import domain.DomainController;
import domain.ItemType;

public class OverviewWindow extends BorderPane{

    @FXML
    private Button btnAddWeapon;
    @FXML
    private Button btnAddKey;
    @FXML
    private TextArea txaOverview;

    private DomainController dc;

        this.dc = dc;

        FXMLLoader loader = new FXMLLoader(getClass().getResource("OverviewWindow.fxml"));
        loader.setRoot(this);
        loader.setController(this);
        try{
            loader.load();
            txaOverview.setText(dc.showOverview()); // showOverview returns a String containing a toString of all items in the inventory
        }
        catch (IOException ex){
            throw new RuntimeException(ex);
        }
    }

    @FXML
    public void btnAddWeaponOnAction(ActionEvent event){
        try{
            add(ItemType.WEAPON); // ItemType is an Enum where all the properties of the items are defined; for Weapon: name, weight, level, power, used(boolean)
        }
        catch (Exception e){
            throw e;
        }
    }

    @FXML
    public void btnAddKeyOnAction(ActionEvent event){
        try{
            add(ItemType.SLEUTEL); // ItemType is an Enum where all the properties of the items are defined; for Key: name, weight, level, door
        }
        catch (Exception e){
            throw e;
        }
    }

    private void add(ItemType itemType){
        Stage stage = new Stage();
        stage.setTitle(itemType== VoorwerpSoort.WEAPON ? "Add Weapon" : "Add Key");
        AddItem addItem = new AddItem(dc,itemType,this);

        addItem.setOnHiding(new EventHandler<WindowEvent>(){
            @Override
            public void handle(WindowEvent e){
                txaOverview.setText(dc.showOverview()); // when we close the AddItemWindow, we will update the item overview by having the domain controller get this data from the repository
            }
        });

        addItem.show();
    }
}

/* AddItemWindow, a subclass of Stage*/ package gui;

// import relevant javafx and java classes
import domain.DomainController;
import domain.ItemType;

public class AddItemWindow extends Stage {

    @FXML
    private BorderPane addRoot;
    @FXML
    private Button btnOK;
    @FXML
    private Button btnCancel;

    private DomainController dc;
    private ItemType itemType;
    private Parent parent;

    private TextField[] txfAttributes;

    public AddItemWindow(DomainController dc, ItemType itemType, OverviewWindow overviewWindow){
        this.dc = dc;
        this.itemType = itemType;
        this.parent = overviewWindow;
        this.setScene(buildGUI(dc,itemType,overviewWindow));
    }

    private Scene buildGUI(DomeinController dc, VoorwerpSoort vwps, OverzichtSchermController ovsController){
        Parent root = new BorderPane();

        GridPane properties = new GridPane();

        properties.setPadding(new Insets(10));
        properties.setHgap(10);
        properties.setVgap(10);

        ColumnConstraints col1 = new ColumnConstraints();
        col1.setHalignment(HPos.RIGHT);
        ColumnConstraints col2 = new ColumnConstraints();
        properties.getColumnConstraints().addAll(col1, col2);

        String[] attributes = itemType.attributeNames();
        txfAttributes = new TextField[attributes.length];

        for(int i = 0; i<attributes.length; i++){
            properties.add(new Label(attributes[i]),0,i);
            properties.add(txfAttributes[i] = new TextField(),1,i);
        }

        ((BorderPane) root).setCenter(properties);

        FXMLLoader loader = new FXMLLoader(getClass().getResource("AddItemWindow.fxml"));
        loader.setRoot(root);
        loader.setController(root);
        try{
            loader.load();
            return new Scene(root);
        }
        catch(IOException e){
            throw new RuntimeException(e);
        }
    }


    // NOT WORKING
    @FXML
    public void btnOKOnAction(){
        addItem();
    }

    // NOT WORKING
    @FXML
    public void btnCancelOnAction(ActionEvent event){
        hide();
    }

    private void voorwerpToevoegen(){ // we're calling the domaincontroller to add the new item to the repository
        switch (itemType)
        {
    // for the item, add an item by getting the value of each TextField, which are the
    // parameters for a constructor of the new item           
            case WEAPON:
                dc.addWeapon(txfAttributes[0].getText(),
                    Double.parseDouble(txfAttributes[1].getText()),
                    Integer.parseInt(txfAttributes[2].getText()),
                    Integer.parseInt(txfAttributes[3].getText()),
                    Boolean.parseBoolean(txfAttributes[4].getText()));
                break;
            case KEY:
                dc.addKey(txfAttributes[0].getText(),
                    Double.parseDouble(txfAttributes[1].getText()),
                    Integer.parseInt(txfAttributes[2].getText()),
                    Integer.parseInt(txfAttributes[3].getText()));
                break;
        }
        hide();
    }
}