[Javafx] Prendere componenti nel file fxml

[fermat]

ciao!
mi sto affacciando a javafx e riscontro un problema tra controller e file fxml.
in pratica nel file fxml ho questo:

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<AnchorPane id="AnchorPane" prefHeight="162.0" prefWidth="300.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" 
            fx:controller="com_mp_bf.FXMLDocumentController">
    <children>
        <Button fx:id="buttonBook" layoutX="14.0" layoutY="15.0" mnemonicParsing="false" onAction="#launchBook" prefHeight="128.0" prefWidth="128.0" />
        <Button fx:id="buttonFilm" layoutX="161.0" layoutY="15.0" mnemonicParsing="false" onAction="#launchFilm" prefHeight="128.0" prefWidth="128.0" />
    </children>
</AnchorPane>

poi nel controller:

public class FXMLDocumentController implements Initializable {

    @FXML
    private Button buttonFilm;
    private Button buttonBook;
    
    @FXML
    private void launchBook(ActionEvent event) {
    }

    @FXML
    private void launchFilm(ActionEvent event) {
    }

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        Image imageBook = new Image(".............");
        Image imageFilm = new Image("...............");
        buttonBook.setGraphic(new ImageView(imageBook)); --> NullPointerException
        buttonFilm.setGraphic(new ImageView(imageFilm));
    }

}

però ottengo un errore (ho segnato la riga dove lo da):

Caused by: java.lang.NullPointerException
    at com_mp_bf.FXMLDocumentController.initialize(FXMLDocumentController.java:34)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2542)
    ... 22 more

sinceramente non riesco a capire dove sbaglio!