Bug in Java Client code when calling AWT code from a JavaFX context

ClientBug.java

import java.io.*;
import java.net.URL;
import java.nio.file.*;

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.HBox;
import javafx.scene.control.Button;

public class ClientBug extends Application {

	public static void main (String[] args) {
		launch(args);
		// loadDocAWT();
		// System.out.println("once");
		// loadDocAWT();
		// System.out.println("twice");
		// loadDocAWT();
		// System.out.println("thrice");
	}

	@Override
	public void start(Stage stage) {
		Button btn = new Button("Click Me");
		btn.setOnAction( ae -> loadDocAWT() );

		HBox root = new HBox(btn);
		Scene scene = new Scene(root, 400, 400);
		stage.setScene(scene);
		stage.show();
	}

	private static void loadDocAWT() {
		try {
			// File pdf = File.createTempFile("sample", ".pdf");
			File pdf = new File ("sample.pdf");			

			InputStream in = new URL("https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf").openStream();
			Files.copy(in, pdf.toPath(), StandardCopyOption.REPLACE_EXISTING);

			System.out.println("before execution");
			java.awt.Desktop.getDesktop().open(pdf);
			System.out.println("successfully executed");
		} catch (IOException ioe) {
			//log
			System.err.println("An I/O Error occurred in loading PDF data.");
		}
	}

}