widthProperty nullPointerException при добавлении слушателя JavaFX

Question

Я хочу изменить root сцены, когда она достигает определенной ширины. Поэтому я создал прослушиватель изменений, в котором я изменяю root, и я пытаюсь добавить прослушиватель изменений в свойство width сцены, но я получаю исключение nullPointerException во время выполнения. Я не могу понять, почему это происходит, я пробовал со сценой, но я получаю ту же ошибку.

Это код:

import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;

import javafx.scene.control.Button;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.stage.Stage;
import javafx.stage.Modality;
import javafx.scene.Scene;

import javafx.event.EventHandler;
import javafx.event.ActionEvent;
import javafx.beans.value.ChangeListener;

import java.io.IOException;

import java.util.ArrayList;
import java.util.List;
import javafx.collections.ObservableList;
import javafx.collections.FXCollections;

import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.value.ObservableValue;
import java.lang.Number;

public class MainController
{
    @FXML Button infoButton;
    @FXML Button aboutButton;
    @FXML ComboBox<Integer> numberCombo;
    @FXML Label symbolLabel;
    @FXML Label nameLabel;
    @FXML Label massLabel;
    @FXML Label ceLabel;

    Scene mainScene;

    SimpleIntegerProperty selectedElement = new SimpleIntegerProperty();

    ObservableList<Integer> numberOList;
    List<Integer> numberList = new ArrayList();

    EventHandler<ActionEvent> buttonClick;
    ChangeListener<Number> selectedChange;
    ChangeListener<Number> widthChange;


    @FXML
        public void initialize()
        {
        setHandlers();
        mainScene = infoButton.getScene();
        mainScene.widthProperty().addListener(widthChange); //ERROR OCURRS HERE <-----
        infoButton.addEventHandler(ActionEvent.ACTION, buttonClick);
        aboutButton.addEventHandler(ActionEvent.ACTION, buttonClick);
        Table.createTable();
        for(int i = 0; i < Table.getTable().size(); i++)
            numberList.add(Integer.parseInt(Table.getElement(i).getNumber()));
        numberOList = FXCollections.observableList(numberList);
        numberCombo.setItems(numberOList);
        numberCombo.getSelectionModel().selectFirst();
        setSelectedElement(0);
        numberCombo.addEventHandler(ActionEvent.ACTION, buttonClick);
        selectedElement.addListener(selectedChange);

        }

    public void setHandlers()
    {
        buttonClick = new EventHandler<ActionEvent>()
        {
            @Override
            public void handle(ActionEvent event)
            {
                FXMLLoader loader = new FXMLLoader();
                try
                {
                    if(event.getSource() == infoButton)
                    {
                        loader.setLocation(getClass().getResource("\\layout\\info.fxml"));
                        loader.load();
                        InfoController controller = loader.getController();
                        controller.setSelectedElement(selectedElement.get());
                        infoButton.getScene().setRoot(loader.getRoot());
                        System.out.println("Info button clicked...");
                    }
                    else if(event.getSource() == aboutButton)
                    {

                        Stage aboutStage = new Stage();
                        aboutStage.setScene(new Scene(FXMLLoader.load(getClass().getClassLoader().getResource("\\layout\\about.fxml"))));
                        aboutStage.setResizable(false);
                        aboutStage.setWidth(400);
                        aboutStage.setHeight(250);
                        aboutStage.initModality(Modality.WINDOW_MODAL);
                        aboutStage.initOwner((Stage) infoButton.getScene().getWindow());
                        aboutStage.show();
                        System.out.println("About button clicked...");
                    }
                    else if(event.getSource() == numberCombo)
                    {
                        setSelectedElement(numberCombo.getValue()-1);
                    }
                }
                catch(IOException exception)
                {
                    exception.printStackTrace();
                }
            }
        };

        selectedChange = new ChangeListener<Number>()
        {
            @Override
            public void changed(ObservableValue<? extends Number> observable, Number oldValue, Number newValue)
            {
                numberCombo.getSelectionModel().select(selectedElement.get());
                symbolLabel.setText(Table.getElement(selectedElement.get()).getSymbol());
                nameLabel.setText(Table.getElement(selectedElement.get()).getName());
                massLabel.setText(Table.getElement(selectedElement.get()).getMass());
                ceLabel.setText(Table.getElement(selectedElement.get()).getCE());
            }
        };

        widthChange = new ChangeListener<Number>() //CHANGE LISTENER FOR THE WIDTH PROPERTY
        {
            FXMLLoader loader = new FXMLLoader();

            @Override
            public void changed(ObservableValue<? extends Number> observable, Number oldValue, Number newValue)
            {
                if(newValue.doubleValue() >= Double.valueOf(600))
                {
                    try
                    {
                        loader.setLocation(getClass().getResource("\\layout\\widemain.fxml"));
                        loader.load();
                        infoButton.getScene().setRoot(loader.getRoot());
                    }
                    catch(IOException exception)
                    {
                        exception.printStackTrace();
                    }
                }
            }
        };
    }

    public void setSelectedElement(int selectedElement)
    {
        this.selectedElement.set(selectedElement);
    }

    public void setSelectedElement(SimpleIntegerProperty selectedElement)
    {
        this.selectedElement = selectedElement;
    }
}

И это исключение, которое он бросает :

 at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2601)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2571)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2441)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3214)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3175)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3148)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3124)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3104)
        at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3097)
        at Main.start(Main.java:16)
        at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$8(LauncherImpl.java:863)
        at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$7(PlatformImpl.java:326)
        at com.sun.javafx.application.PlatformImpl.lambda$null$5(PlatformImpl.java:295)
        at java.security.AccessController.doPrivileged(Native Method)
        at com.sun.javafx.application.PlatformImpl.lambda$runLater$6(PlatformImpl.java:294)
        at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.lambda$null$4(WinApplication.java:186)
        at java.lang.Thread.run(Unknown Source) Caused by: java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at sun.reflect.misc.Trampoline.invoke(Unknown Source)
        at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
        at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2566)
        ... 17 more Caused by: java.lang.NullPointerException
        at MainController.initialize(MainController.java:53)
        ... 27 more