NullPointerException error while running my program in IDE

Question

I have a weird problem. My IDE says everthing about my code is alright but when I try to run it, it says an error has occured and the program doesn't run. I've read through the code again but I simply don't know what to do to get it working. I'm hoping you could help me out with this issue. Thanks in advance.

  package twobuttons;

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;

public class TwoButtons extends JFrame implements ActionListener {

    JButton button1;
    JButton button2;
    JLabel label;

    public static void main(String[] args){
     new TwoButtons();


    }
    public TwoButtons(){
        this.setTitle("Dos botones");
        this.setSize(400,400);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setLayout(new FlowLayout());

        button1.setText("hola");
        button1.addActionListener(this);
        button1.setActionCommand("hola");

        button2.setText("Te odio.");
        button2.addActionListener(this);
        button2.setActionCommand("Te odio.");

        button1.add(this);
        button2.add(this);
        this.setVisible(true);


    } 


    @Override
    public void actionPerformed(ActionEvent e) {
        String command= e.getActionCommand();

        if (e.getActionCommand().equals("hola")){
           JOptionPane.showMessageDialog(this.getComponent(0), "Espero que tengas un buen día");  
        } else {
            JOptionPane.showMessageDialog(this.getComponent(0), "Odio mi vida");
        }
    }}

And the error I get is:

Exception in thread "main" java.lang.NullPointerException
    at twobuttons.TwoButtons.<init>(TwoButtons.java:28)
    at twobuttons.TwoButtons.main(TwoButtons.java:18)
C:\Users\Juan\Desktop\Informatik 12\TwoButtons\nbproject\build-impl.xml:1040: The following error occurred while executing this line:
C:\Users\Juan\Desktop\Informatik 12\TwoButtons\nbproject\build-impl.xml:805: Java returned: 1
BUILD FAILED (total time: 1 second)