JavaFX Panel inside Panel auto resizing

I have a JavaFX application which has only one FXML file. In this file I have one AnchorPane which has a StackPane inside it. Here is the screenshot:

When I start this application, I want to resize StackPane automatically with the AnchorPane. Thus; StackPane will get the current avaliable width and height automatically. At the moment when I resize the application, AnchorPane is being resized automatically but StackPane stays as his fixed size.

How can I resize the StackPane automatically and make it fully stretched inside its parent panel?

My Code

Main.java

package app;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

  public static void main(String[] args) {
      Application.launch(args);
  }

  @Override
  public void start(Stage stage) throws Exception {
      Parent root = FXMLLoader.load(getClass().getResource("Main.fxml"));
      Scene scene = new Scene(root,800,600);
      scene.getStylesheets().add(this.getClass().getResource("/app/style1.css").toExternalForm());
      stage.setScene(scene);
      stage.show();
  }
}

MainController.java

package app;

import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.StackPane;

public class MainController implements Initializable {

   @FXML
   private AnchorPane anchorPane;
   @FXML
   private StackPane stackPane;

   @Override
   public void initialize(URL url, ResourceBundle rb) {
     stackPane.setPrefSize(anchorPane.getPrefWidth(), anchorPane.getPrefHeight()); //didn't work
   }    
}

Main.fxml

<