setImage ImageView outside initialize() NullPointerException

Question

I am a new javafx user. I'm trying to create a basic application but I'm having some problems. I try, when closing a sub-window, to change the icons of my buttons by changing the Image inside my ImageView.

I have my main window :

<AnchorPane id="AnchorPane" prefHeight="651.0" prefWidth="823.0" 
   xmlns="http://javafx.com/javafx/10.0.1" 
   xmlns:fx="http://javafx.com/fxml/1" 
     fx:controller="amzevent.UI.FXMLMainUI">
    <children>
      <ScrollPane layoutX="14.0" layoutY="110.0" prefHeight="526.0" 
       prefWidth="796.0" />
        <ToolBar layoutX="14.0" layoutY="14.0" prefHeight="46.0" 
         prefWidth="796.0">
          <items>
            <Button minHeight="40.0" minWidth="40.0" mnemonicParsing="false" 
             prefHeight="34.0" prefWidth="40.0">
           <graphic>
              <ImageView fitHeight="30.0" fitWidth="30.0" 
               pickOnBounds="true" preserveRatio="true" 
               fx:id="bouton_retour">
                 <image>
                    <Image url="@../../resources/undo (1).png"   />
                 </image>
              </ImageView>
           </graphic>
           <tooltip>
              <Tooltip text="Annuler" />
           </tooltip>
        </Button>
     </items>
  </ToolBar>
  </children>
  </AnchorPane>

and the controller :

public class FXMLMainUI extends Controlleur implements Initializable {



@FXML 
private ImageView bouton_retour; 

@FXML
protected void ClickBoutonCreerSalle(ActionEvent event) {
    System.out.println("JavaMainUI : Click bouton créer salle");
    try {

        FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("FXMLCreerSalle.fxml"));
        Parent root1 = (Parent) fxmlLoader.load();
        this.salle = new Stage();
        this.salle.setTitle("Nouvelle Salle");
        this.salle.setScene(new Scene(root1));
        this.salle.show();
    } catch (IOException ex) {
        Logger.getLogger(FXMLMainUI.class.getName()).log(Level.SEVERE, null, ex);
    }
    /*FXMLLoader loader = new FXMLLoader(getClass().getResource("FXMLMainUI.fxml"));
    try {
        Parent root = loader.load();
    } catch (IOException ex) {
        Logger.getLogger(FXMLMainUI.class.getName()).log(Level.SEVERE, null, ex);
    }*/
}


protected void CreerSalle(String nom_salle, int largeur_salle, int profondeur_salle) {
    super.CreerSalle(nom_salle, largeur_salle, profondeur_salle);
    System.out.println("JavaMainUI : Créer salle");
}


@FXML
protected void MettreBoutonCliquables() {
    System.out.println("JavaMainUI : MettreBoutonCliquables");
    Image img = new Image("resources/undo.png");
    this.bouton_retour.setImage(img);
}

@Override
public void initialize(URL url, ResourceBundle rb) {
    assert this.bouton_retour != null : "fx:id=\"bouton_retour\" was null check your FXML "; 
    this.bouton_retour = new ImageView();
    System.out.println("JavaMainUI : Initialize ");
}    
public FXMLMainUI()
{
}

}

the other view :

   <AnchorPane id="AnchorPane" prefHeight="400.0" prefWidth="600.0" 
   xmlns="http://javafx.com/javafx/10.0.1" 
   xmlns:fx="http://javafx.com/fxml/1" 
   fx:controller="amzevent.UI.FXMLCreerSalle">
    <children>
    <Button layoutX="486.0" layoutY="326.0" mnemonicParsing="false" 
       text="Button" onAction="#SoumettreNouvelleSalle" />
    <Label layoutX="118.0" layoutY="132.0" prefHeight="17.0" 
     prefWidth="59.0" text="Nom" />
     <Label layoutX="116.0" layoutY="183.0" prefHeight="17.0" 
   prefWidth="65.0" text="Largeur" />
    <TextField layoutX="208.0" layoutY="128.0" fx:id="field_nom"/>
    <TextField layoutX="208.0" layoutY="179.0" fx:id="field_largeur"/>
    <TextField layoutX="208.0" layoutY="233.0" fx:id="field_profondeur"/>
    <Label layoutX="116.0" layoutY="237.0" prefHeight="17.0" 
     prefWidth="65.0" 
  text="Profondeur" />
    <Label layoutX="363.0" layoutY="183.0" text="en metre" />
    <Label layoutX="363.0" layoutY="237.0" text="en metre" />
  </children>
  </AnchorPane> 

the other controller :

  public class FXMLCreerSalle extends FXMLMainUI implements Initializable {

@FXML
private TextField field_nom;

@FXML
private TextField field_largeur;

@FXML
private TextField field_profondeur;



@FXML
protected void SoumettreNouvelleSalle(ActionEvent event) {
    System.out.println("JavaMainUI : Soumettre nouvelle salle");
    if (!this.field_nom.getText().equals("") && !this.field_largeur.getText().equals("") && !this.field_profondeur.getText().equals("")) {
        String nom_salle = this.field_nom.getText();
        int largeur_salle = 0;
        int profondeur_salle = 0;
        try {
            largeur_salle = Integer.parseInt(this.field_largeur.getText());
            profondeur_salle = Integer.parseInt(this.field_profondeur.getText());
        }  catch (NumberFormatException e) {
            System.out.println("JavaMainUI : largeur et profondeur doivent etre des nombre entiers"); 
            return;
        }
        System.out.println("JavaMainUI : " + nom_salle); 
        System.out.println("JavaMainUI : " + largeur_salle);
        System.out.println("JavaMainUI : " + profondeur_salle);
        Stage stage = (Stage) this.field_nom.getScene().getWindow();
        // do what you have to do
        stage.close();
        super.CreerSalle(nom_salle, largeur_salle, profondeur_salle);
        super.MettreBoutonCliquables();
    }
}

@Override
public void initialize(URL url, ResourceBundle rb) {

    // TODO
    //retour_couleur = new Image("resources/undo.png");
    //this.retour = new ImageView();
    //this.boutton_retour = new ImageView();
}    

}

when the sub windows closes, it calls super.MettreBoutonCliquables(); to change the Image of the ImageView with a new source.

When i run my program everything works fine until this.bouton_retour.setImage(img); -> java.lang.NullPointerException

I tried to add the followed lines to reload my fxml main file but didn't change anything. FXMLLoader loader = new FXMLLoader(getClass().getResource("/amzevent/UI/FXMLMainUI.fxml")); Parent root = loader.load();

In my initialize funtion the bouton_retour is not null.

I have no idea how to fix this, If anyone's ever had this kind of problem before, I'm a taker.