Thread: JavaFX NullPointerException when creating a new stage.

Hi guys,
I'm fairly new to JavaFX and I've been trying to create multiple stages in my project.

So what I currently have is a main java class to open a Login Page under the Start Method, a Login Controller to open a Main Page and a Main Controller to open a new stage file called "Quotation FXML.fxml".
The Login page and the main page opens fine, but when I tried to open a Quotation FXML.fxml stage using a button, it gives me a NPE error.

//Initialise controller
    @Override
    public void initialize(URL url, ResourceBundle rb) {
 
            pnClient.toFront();
            txtID.setText("ID");
            txtName.setText("Clients");
            txtMenuName.setText("/home/clients");
            txtClientName.setText("Client Name");
            txtLoc.setText("Location");
            txtWS.setText("W/S Fee");
            txtMat.setText("Material");
            txtMD.setText("MD/SP Fee");
            txtfEle.setVisible(false);
            txtfFee.setVisible(false);
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(229, 8, 8), CornerRadii.EMPTY, Insets.EMPTY )));
 
 
        try {   
            UpdateTable();
            searchClients();
            UpdateElement();
            searchElements();
        } catch (SQLException ex) {
 
        }
    }
 
 
    //adding pane title and colour to categories
    @FXML
    public void handleClicks (ActionEvent event) throws SQLException {       
 
        if (event.getSource() == btnNew) {
            pnClient.toFront();
            txtID.setText("ID");
            txtName.setText("Clients");
            txtMenuName.setText("/home/clients");
            txtClientName.setText("Client Name");
            txtLoc.setText("Location");
            txtWS.setText("W/S Fee");
            txtMat.setText("Material");
            txtMD.setText("MD/SP Fee");
            txtfLoc.setVisible(true);
            txtfWS.setVisible(true);
            txtfMat.setVisible(true);
            txtfMD.setVisible(true);
            txtLoc.setVisible(true);
            txtWS.setVisible(true);
            txtMat.setVisible(true);
            txtMD.setVisible(true);
            txtfID.setVisible(true);
            txtfClientName.setVisible(true);
            txtfEle.setVisible(false);
            txtfFee.setVisible(false);
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(229, 8, 8), CornerRadii.EMPTY, Insets.EMPTY )));
 
        }
 
        else if (event.getSource() == btnMat) {
            txtName.setText("Material Detail");
            txtMenuName.setText("/home/materials");
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(0, 150, 209), CornerRadii.EMPTY, Insets.EMPTY )));
 
        }
        else if (event.getSource() == btnSavedQ) {
            pnSavedQ.toFront();
            txtName.setText("Saved Quotation");
            txtMenuName.setText("/home/saved");
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(255, 131, 0), CornerRadii.EMPTY, Insets.EMPTY )));
 
        }
        else if (event.getSource() == btnPrintQ) {
            txtName.setText("Print Quotation");
            txtMenuName.setText("/home/print");
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(170, 65, 246), CornerRadii.EMPTY, Insets.EMPTY )));
 
        }
        else if (event.getSource() == btnAss) {
            pnAss.toFront();
            txtName.setText("Assay Detail");
            txtMenuName.setText("/home/assay elements");
            txtID.setText("Fee");
            txtClientName.setText("Material Name");
            txtfLoc.setVisible(false);
            txtfWS.setVisible(false);
            txtfMat.setVisible(false);
            txtfMD.setVisible(false);
            txtLoc.setVisible(false);
            txtWS.setVisible(false);
            txtMat.setVisible(false);
            txtfID.setVisible(false);
            txtfClientName.setVisible(false);
            txtMD.setVisible(false);
            txtfEle.setVisible(true);
            txtfFee.setVisible(true);
            paneTitle.setBackground(new Background(new BackgroundFill(Color.rgb(48, 168, 124), CornerRadii.EMPTY, Insets.EMPTY )));
 
        }
    }
 
    @FXML
    public void setID (ActionEvent event) {
        String ID = txtfClientName.getText();
        ID = ID.replaceAll("\\s+","");
        ID = ID.toUpperCase();
        ID = ID.substring(0,4);
        ID = ID+"01"+"KR";
        txtfID.setText(ID);
    }
 
 
 public void createQuotationForm(ActionEvent event) throws IOException {
    FXMLLoader main = new FXMLLoader();
    main.setLocation(getClass().getResource("/LoginFX/QuotationFXML.fxml"));
    Parent mainParent = main.load();
    Scene mainScene = new Scene(mainParent);
 
    Scene currentScene = mainParent.getScene();
    Stage stage = (Stage) currentScene.getWindow();
 
    stage.setScene(mainScene);
 }

 
javafx.fxml.LoadException: 
/Users/seokjuhan/NetBeansProjects/Login_App/build/classes/LoginFX/QuotationFXML.fxml
 
    at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2707)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2685)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548)
    at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2517)
    at Login_App/LoginFX.MainFXMLController.createQuotationForm(MainFXMLController.java:571)
    at Login_App/LoginFX.MainFXMLController.doubleclick(MainFXMLController.java:562)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1857)
    at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1729)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3856)
    at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1851)
    at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2584)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:409)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:299)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:447)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:446)
    at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
    at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
    at javafx.graphics/com.sun.glass.ui.mac.MacView.notifyMouse(MacView.java:127)
Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.layout.GridPane.toFront()" because "this.pnClient" is null
    at Login_App/LoginFX.MainFXMLController.initialize(MainFXMLController.java:451)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2655)
    ... 41 more

So this is the part of the code I have on my Main Controller. I think I understand why I'm getting NPE, it's because I'm trying to load QuotationFXML which doesn't have the elements such as pnClient, txtID and etc., that I initialised. So I thought maybe I need to have this createQuotationForm() code in a new Controller. My problem though is that, I have the button, which should opens the QuotationFXML file, on my MainFXML.fxml so I need to have the method createQuotationForm(ActionEvent event), in my Main Controller. I'm just out of ideas how to get around this problem.
Do I need to get rid of all the initialisers?

Any help would be great.

Thank you.

because "this.pnClient" is null

Where is pnClient given a value? It looks like the code is being executed with pnClient having a null value.

I don't do JavaFX coding and haven't seen many of its errors.
If you can make a small, complete program that compiles and executes for testing, I can probably find the problem.
The posted code is from a very large file that is to big to post here for testing.