Appending text to a listview in JFX

Question

I'm having trouble appending a username to a listview. I create users in one controller, while calling a method from another controller which appends the newly created username to a listview, but there's multiple runtime errors.

Method of adding users in the first controller:

void addUser(ActionEvent event) {
    User user = new User();
    user.setUsername(newUserField.getText());
    user.setPassword(newPassField.getText());
    UserDAO theDAO = new UserDAO();
    theDAO.createUser(user);
    MainPanelAdminController mc = new MainPanelAdminController();
    mc.appendList(user.getUsername())
}

And this is the append method in the second controller:

void appendList(String username) {
    Label lbl = new Label(username);
    ObservableList<Label> items = FXCollections.observableArrayList(lbl);
    listView.setItems(items);
}

Note: The setUsername(), setPassword(), createUser() and getUsername() methods are all working; the issue comes about at listView.setItems(items) - I added debugging lines to check.

Right now when I add a user it gives this error:

Caused by: java.lang.NullPointerException at Admin.panel.MainPanelAdminController.appendList(MainPanelAdminController.java:61) at Admin.panel.createUserController.addUser(createUserController.java:35) ... 58 more

Line 61: listView.setItems(items); Line 35: mc.appendList(user.getUsername());