can't execute controller method after loading fxml document

Question

i'm trying to add an AnchorPane to my fxml file, after loading it in main through a method i defined in the controller, but i'm getting this error

Exception in Application start method
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 com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    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.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
    at sample.Controller.addAnchor(Controller.java:20)
    at sample.Main.start(Main.java:16)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    ... 1 more
Exception running application sample.Main

Controller.java

public class Controller {
    VBox vBox;
    public void addAnchor(){
        AnchorPane user = new AnchorPane();
        user.setPrefHeight(50);
        user.setPrefWidth(200);
        Label test = new Label("test");
        test.setLayoutX(113);
        test.setLayoutY(15);
        test.setCursor(Cursor.HAND);
        user.setCursor(Cursor.HAND);
        user.getChildren().add(test);
        vBox.getChildren().add(user);
    }
}

Main.java

@Override
    public void start(Stage primaryStage) throws Exception{
        FXMLLoader loader = new FXMLLoader(getClass().getResource("sample.fxml"));
        loader.setController(new Controller());
        Controller controller = loader.getController();
        controller.addAnchor();
        Parent root = loader.load();
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 800, 600));
        primaryStage.show();
    }

GridPane

<GridPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="572.0" prefWidth="755.0" xmlns="http://javafx.com/javafx/10.0.2-internal" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">

Vbox

<VBox id="vBox" maxHeight="1.7976931348623157E308" prefHeight="444.0" prefWidth="252.0" style="-fx-background-color: #C3B091;">

Project structure

I've been stuck here for a day, any help would be very much appreciated.