Populating an ObservableList<String> from a List<CLASS> in JavaFx

In my controller class I have private ObservableList<String> options = FXCollections.observableArrayList(); which it will be used to populate a ComboBox (eg: comboBox.setItems(options);). I am trying to retrieving the various options from an XML file.

This is my method for reading the XML:

package dima.configuration.view;

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.IntStream;

import dima.configuration.tools.TestBus;
import javafx.collections.ObservableList;
import javafx.fxml.FXML;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.stage.FileChooser;
import dima.configuration.MainApp;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;

/**
 * The controller for the root layout. The root layout provides the basic
 * application layout containing a menu bar and space where other JavaFX
 * elements can be placed.
 *
 *
 */
public class RootLayoutController {

    // Reference to the main application
    private MainApp mainApp;

    private ConfigurationEditDialogController configurationEditDialogController;

    private static List<TestBus> testBuses = new ArrayList<>();

    public List<TestBus> getTestBuses(){
        return testBuses;
    }



    /**
     * Is called by the main application to give a reference back to itself.
     *
     * @param mainApp
     */
    public void setMainApp(MainApp mainApp) {
        this.mainApp = mainApp;
    }

    /**
     * Creates an empty set of configurations.
     */
    @FXML
    private void handleNew() {
        mainApp.getConfigurationData().clear();
        mainApp.setConfigurationFilePath(null);
    }

    /**
     * Opens a FileChooser to let the user select a set of configurations to load.
     */
    @FXML
    private void handleOpen() {
        FileChooser fileChooser = new FileChooser();

        // Set extension filter
        FileChooser.ExtensionFilter extFilter = new FileChooser.ExtensionFilter(
                "XML files (*.xml)", "*.xml");
        fileChooser.getExtensionFilters().add(extFilter);

        // Show save file dialog
        File file = fileChooser.showOpenDialog(mainApp.getPrimaryStage());

        if (file != null) {
            mainApp.loadConfigurationDataFromFile(file);
        }
    }


    @FXML
    private void loadTestBuses(){
        try {

            FileChooser fileChooser = new FileChooser();

            // Set extension filter
            FileChooser.ExtensionFilter extFilter = new FileChooser.ExtensionFilter(
                    "XML files (*.xml)", "*.xml");
            fileChooser.getExtensionFilters().add(extFilter);

            // Show save file dialog
            File fXmlFile = fileChooser.showOpenDialog(mainApp.getPrimaryStage());

            if (fXmlFile != null) {


                DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
                DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
                Document doc = dBuilder.parse(fXmlFile);
                doc.getDocumentElement().normalize();

                NodeList testBusNodeList = doc.getElementsByTagName("testbus");

                for (int parameter = 0; parameter < testBusNodeList.getLength(); parameter++) {
                    TestBus testBus = new TestBus();
                    Node node = testBusNodeList.item(parameter);
                    if (node.getNodeType() == Node.ELEMENT_NODE) {
                        Element eElement = (Element) node;
                        String index = eElement.getElementsByTagName("index").item(0).getTextContent();
                        String tb_name = eElement.getElementsByTagName("tb_name").item(0).getTextContent();
                        NodeList bitf_name = eElement.getElementsByTagName("bitf_name");
                        List<String> bitf_namesList = new ArrayList<>();
                        IntStream.range(0, bitf_name.getLength()).forEach(bName -> {
                            bitf_namesList.add(bitf_name.item(bName).getTextContent());
                        });
                        testBus.setIndex(index);
                        testBus.setTb_name(tb_name);
                        testBus.setBitf_names(bitf_namesList);

                        testBuses.add(testBus);
                    }
                }
            }
            } catch(Exception e){
                System.out.println("!!!!!!!!  Exception while reading xml file :" + e.getMessage());
            }

            configurationEditDialogController.options.add(0, testBuses.get(0).getTb_name());

            System.out.println("###################################################");
            System.out.println(testBuses);


    }
}

This is the TestBus class

public class TestBus {
private String index;
private String tb_name;
private List<String> bitf_names;

    public String getIndex() {
        return index;
    }

    public void setIndex(String index) {
        this.index = index;
    }

    public String getTb_name() {
        return tb_name;
    }

    public void setTb_name(String tb_name) {
        this.tb_name = tb_name;
    }

    public List<String> getBitf_names() {
        return bitf_names;
    }

    public void setBitf_names(List<String> bitf_name) {
        this.bitf_names = bitf_name;
    }

    @Override
    public String toString() {
        return "TestBus [index=" + index + ", tb_name=" + tb_name + ", bitf_name=" + bitf_names + "]";
    }
}

I tried to populate the options ObservableList<String> using configurationEditDialogController.options.add(0, testBuses.get(0).getTb_name()); and iterating around i until testBuses.size(). However I get NullPointerException on the above line, even if I try to add only one item. I am also sure that the reading from XML works because the loadTestBuses method prints the right output. I do not get it, maybe I am trying to populate it in the wrong way.

NPE

    Jul 27, 2018 5:09:22 PM java.util.prefs.WindowsPreferences <init>
WARNING: Could not open/create prefs root node Software\JavaSoft\Prefs at root 0x80000002. Windows RegCreateKeyEx(...) returned error code 5.
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
    at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1405)
    at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$343(ContextMenuContent.java:1358)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:394)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$353(GlassViewEventHandler.java:432)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1771)
    ... 43 more
Caused by: java.lang.NullPointerException
    at dima.configuration.view.RootLayoutController.loadTestBuses(RootLayoutController.java:132)
    ... 53 more

EDIT:

I have added the Stack from the console and also the class in a proper way.