A bound value cannot be set.

Bonjour jais une erreur ( A bound value cannot be set ) quand je lence mon application.

Normalement j"utillise Group root = new Group(); mais cette fois dans mon application je dois metre AnchorPane root = new AnchorPane();
Mais quand j"utillise le Group tout fonctionne aucune erreur. Alors pourquoi jais une erreur ( A bound value cannot be set ) quand j"utillise AnchorPane ^^^^ :( PLZ Help me

Code:

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package application;
 
import javafx.application.Application;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.AnchorPane;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
 
 
public class Main extends Application {
 public static SimpleDoubleProperty MoouseLocationX = new SimpleDoubleProperty();
 public static SimpleDoubleProperty MoouseLocationY = new SimpleDoubleProperty();
 public static Rectangle DiscutionBackground;
    public static AnchorPane root; 
 public void start(Stage primaryStage) {
  try {
   root = new AnchorPane();
   Scene scene = new Scene(root,400,400);
   scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
   DiscutionBackground = new Rectangle();
   DiscutionBackground.setWidth(40);
   DiscutionBackground.setHeight(40);
   DiscutionBackground.setLayoutX(0);
   DiscutionBackground.setLayoutY(0); 
    AnchorPane.setTopAnchor(DiscutionBackground,scene.heightProperty().get() /3); 
    AnchorPane.setRightAnchor(DiscutionBackground,scene.widthProperty().get() / 3); 
   root.getChildren().addAll(DiscutionBackground); 
   scene.setOnMouseMoved(new EventHandler<MouseEvent>() {
           public void handle(MouseEvent event) {     
              MoouseLocationX.set(event.getSceneX()); 
              MoouseLocationY.set(event.getSceneY());  
          }
         }); 
   Button a = new Button("click");
   a.setOnAction(ActionEvent ->{   
     DiscutionBackground.layoutYProperty().bind(MoouseLocationY);  
            //  DiscutionBackground.layoutXProperty().bind(MoouseLocationX);  
   });
   root.getChildren().add(a);
   primaryStage.setScene(scene);
   primaryStage.show();
  } catch(Exception e) {
    System.out.print("error code 1");
  }
 }
 
 public static void main(String[] args) {
  launch(args);
 }
}