Test to have a direct jpackage call inside the project.

src/main/java/de/kneitzel/JavaFXApp.java

@@ -1,33 +1,42 @@
 package de.kneitzel;
 
 import javafx.application.Application;
+import javafx.application.Platform;
+import javafx.fxml.FXMLLoader;
+import javafx.scene.Parent;
 import javafx.scene.Scene;
 import javafx.scene.control.Button;
+import javafx.scene.layout.AnchorPane;
 import javafx.scene.layout.StackPane;
 import javafx.stage.Stage;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import java.io.IOException;
 
 public class JavaFXApp extends Application {
 	static final Logger logger = LogManager.getLogger(JavaFXApp.class);
 
 	@Override
-	public void start(Stage primaryStage) {
+	public void start(Stage primaryStage) throws IOException {
+
 		primaryStage.setTitle("Hello World!");
+		FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("MainWindow.fxml"));
+		Parent root = fxmlLoader.load();
+		primaryStage.setScene(new Scene(root));
+        primaryStage.show();
+		/*
 		Button btn = new Button();
 		btn.setText("Say 'Hello World'");
 		btn.setOnAction(e -> System.out.println("Hello World!"));
 
 		StackPane root = new StackPane();
 		root.getChildren().add(btn);
-		primaryStage.setScene(new Scene(root, 300, 250));
-		primaryStage.show();
+		*/
 	}
 
 	public static void main(String[] args) {
 		logger.warn("Starting ...");
 		launch(args);
 	}
-	   
 }

src/main/java/de/kneitzel/MainWindow.java

@@ -0,0 +1,30 @@
+package de.kneitzel;
+
+import javafx.event.ActionEvent;
+import javafx.fxml.FXML;
+import javafx.fxml.Initializable;
+import javafx.scene.control.Button;
+import javafx.scene.control.TextField;
+
+import java.net.URL;
+import java.util.ResourceBundle;
+
+public class MainWindow implements Initializable {
+
+    @FXML
+    private TextField textField;
+    public Button button;
+    @Override
+    public void initialize(URL url, ResourceBundle resourceBundle) {
+        if (button == null) {
+            textField.setText("Button is null");
+        } else {
+            textField.setText("Button is not null");
+        }
+    }
+
+    @FXML
+    private void onButtonClick(ActionEvent actionEvent) {
+
+    }
+}

src/main/java/module-info.java

@@ -3,6 +3,7 @@ module FXAppModule {
     requires javafx.graphics;
     requires javafx.controls;
     requires javafx.web;
+    requires javafx.fxml;
     requires org.apache.logging.log4j;
     requires org.apache.logging.log4j.core;
 

src/main/resources/de/kneitzel/MainWindow.fxml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<?import javafx.scene.control.*?>
+<?import javafx.scene.layout.*?>
+
+<AnchorPane prefHeight="127.0" prefWidth="209.0" xmlns="http://javafx.com/javafx/17.0.2-ea" xmlns:fx="http://javafx.com/fxml/1" fx:controller="de.kneitzel.MainWindow">
+   <children>
+      <Button fx:id="button" layoutX="44.0" layoutY="70.0" mnemonicParsing="false" onAction="#onButtonClick" text="Button" />
+      <TextField fx:id="textField" layoutX="14.0" layoutY="24.0" />
+   </children>
+</AnchorPane>