Problem with GUI on NetBeans

I'm here to see if you can enlighten me with this Java and the GUI I'm just learning.

I'm trying to make a simple 3-section GUI (header, content and footer).

Generate the following code (note, it is not the complete program and I know there are variables that are not used)

package grilla;
import java.awt.*;
import javax.swing.*;
import java.util.*;

public class Remun extends JFrame{

private JFrame ventana;
private JButton sumaHab, sumaDesc;
private JLabel texto1, tituloEnc;
private JCheckBox check1;
private JTextField ingreso1,totalHab,totalDesc,totalAPagar;
private JPanel panelEnc,panelFoot;

public Remun(){
    pantalla();
    encabezado();
    footer();

}

public void encabezado(){
    panelEnc=new JPanel();
    tituloEnc=new JLabel("Sistema de Remuneraciones");
    panelEnc.setLayout(new FlowLayout());
    panelEnc.add(tituloEnc);
}

public void footer(){
    panelFoot=new JPanel();
    sumaHab=new JButton("Sumar Haberes");
    sumaDesc=new JButton("Sumar Descuentos");
    panelFoot.setLayout(new FlowLayout());
    panelFoot.add(sumaHab);
    panelFoot.add(totalHab);
    panelFoot.add(sumaDesc);
    panelFoot.add(totalDesc);
    panelFoot.add(totalAPagar);
}

public void pantalla(){
    ventana=new JFrame("Remuneraciones");
    ventana.setLocationRelativeTo(null);
    ventana.setLayout(new GridLayout());
    ventana.add(panelEnc);
    ventana.add(panelFoot);
    ventana.pack();
    ventana.setVisible(true);
    ventana.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String[] args) {
    Remun g1 = new Remun();
}

}

But when I want to preview the progress this does not run.

This is the error I get.

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1095)
at java.awt.Container.add(Container.java:1007)
at javax.swing.JFrame.addImpl(JFrame.java:567)
at java.awt.Container.add(Container.java:419)
at grilla.Remun.pantalla(Remun.java:46)
at grilla.Remun.<init>(Remun.java:17)
at grilla.Remun.main(Remun.java:54)
Java Result: 1

In case it is important, I attach the versions of Netbeans, JDK and JRE

Product Version: NetBeans IDE 8.0.2 (Build 201411181905)

Java: 1.8.0_191; Java HotSpot (TM) 64-Bit Server VM 25.191-b12

Runtime: Java (TM) SE Runtime Environment 1.8.0_191-b12

Greetings and thanks in advance