Who can help me with this. I cannot seem to implement a popup menu with the JFXPopup class.

LibsysController.java

package sample.systemwindow;

import com.jfoenix.controls.*;
import de.jensd.fx.glyphs.fontawesome.FontAwesomeIconView;
import de.jensd.fx.glyphs.icons525.Icons525;
import de.jensd.fx.glyphs.icons525.Icons525View;
import de.jensd.fx.glyphs.materialdesignicons.MaterialDesignIcon;
import de.jensd.fx.glyphs.materialdesignicons.MaterialDesignIconView;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Region;
import javafx.scene.layout.VBox;
import sample.objectClasses.Book;
import sample.objectClasses.Student;

import java.net.URL;
import java.sql.Timestamp;
import java.util.ResourceBundle;

public class LibsysController implements Initializable{

    public HBox dateLabel;
    public JFXButton removeStudentButton1;
    public JFXButton addStudentButton1;
    public JFXButton addBookButton1;
    public JFXButton removeBookButton1;
    public JFXButton lendBookButton1;
    public JFXHamburger hamburger;
    public JFXPopup popup;
    @FXML
    private Label countLabel;

    @FXML
    private TreeTableColumn<Book, Integer> bookID;

    @FXML
    private TreeTableColumn<Book, String> bookTitle;

    @FXML
    private TreeTableColumn<Book, String> bookISBN;

    @FXML
    private TreeTableColumn<Book, String> bookAuthor;

    @FXML
    private TreeTableColumn<Book, Timestamp> bookDateAdded;

    @FXML
    private JFXListView<String> bookDisciplineListView;

    @FXML
    private JFXTextField searchTextField;

    @FXML
    private JFXButton addStudentButton;

    @FXML
    private JFXButton removeStudentButton;

    @FXML
    private JFXButton addBookButton;

    @FXML
    private JFXButton removeBookButton;

    @FXML
    private JFXButton lendBookButton;

    @FXML
    private JFXListView<String> departmentListView;

    @FXML
    private TreeTableColumn<Student, Integer> studentID;

    @FXML
    private TreeTableColumn<Student, String> studentName;

    @FXML
    private TreeTableColumn<Student, String> studentMatricule;

    @FXML
    private TreeTableColumn<Student, String> studentGender;

    @FXML
    private TreeTableColumn<Student, Integer> studentLevel;

    @FXML
    private TreeTableColumn<Student, Timestamp> studentDateAdded;

    @FXML
    private JFXTextField searchTextField1;

    @FXML
    void searchBooks(ActionEvent event) {

    }

    @FXML
    void searchStudent(ActionEvent event) {

    }

    @Override
    public void initialize(URL location, ResourceBundle resources) {

        popup = new JFXPopup();
        hamburger.addEventHandler((MouseEvent.MOUSE_CLICKED), e->{
            popup.show(hamburger, JFXPopup.PopupVPosition.TOP, JFXPopup.PopupHPosition.LEFT, e.getX(), e.getY());
        });

        //VBox vBox = new VBox();
        //vBox.getChildren().addAll(addStudentButton, removeStudentButton,addBookButton, removeBookButton, lendBookButton);
        //popup.setPopupContent(vBox);
        MaterialDesignIconView icon = new MaterialDesignIconView(MaterialDesignIcon.ACCOUNT_PLUS);
        icon.setStyle("-fx-background-color: #00BCD4");
        addStudentButton.setGraphic(icon);

        MaterialDesignIconView icon1 = new MaterialDesignIconView(MaterialDesignIcon.ACCOUNT_MINUS);
        icon1.setStyle("-fx-background-color: #00BCD4");
        removeStudentButton.setGraphic(icon1);
    }

    public void addStudentAction(ActionEvent actionEvent) {
    }

    public void removeStudentAction(ActionEvent actionEvent) {
    }

    public void addBookAction(ActionEvent actionEvent) {
    }

    public void removeBookAction(ActionEvent actionEvent) {
    }

    public void lendBookButton(ActionEvent actionEvent) {
    }
}