NetBeans IDE 8.1 Couldn't connect to accessibility bus

[ddiggio]

Hi everyone,
i was writing a simple code in Java with NetBeans IDE 8.1 to make a simple GUI but i encountered a problem while running the project;
the code is:

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;

public class GUIQuizMate {

public static void main(String[] args) {
System.out.println("PROVA");
JFrame frame = new JFrame("Quiz Matematica");
JButton button = new JButton("Fai il quiz");
JPanel panel = new JPanel();
panel.add(button);
frame.add(panel);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS E);
frame.setVisible(true);
}
}

The output is:

run:
PROVA

** (java:2196): WARNING **: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-TxusFOJQAt: Connessione rifiutata
BUILD STOPPED (total time: 10 seconds)

I also tried to use the terminal for compile and execute the program and i obtained this:

diggio@diggio-AOD257:~/NetBeansProjects/GUIQuizMate/src/guiquizmate$ javac GUIQuizMate.java
diggio@diggio-AOD257:~/NetBeansProjects/GUIQuizMate/src/guiquizmate$ java GUIQuizMate
Error: unable to find or load the main class GUIQuizMate
diggio@diggio-AOD257:~/NetBeansProjects/GUIQuizMate/src/guiquizmate$

I'm using Xubuntu 17.04
Can someone help me?