Varför visar mitt program thread "main" java.lang.IllegalArgumentException: illegal component position ??

package SelfTest;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class JComboBoxtest extends JFrame implements ActionListener{
    JComboBox lista = new JComboBox();
     JLabel ord = new JLabel();
     JButton del = new JButton("Ta bort");
     JButton add = new JButton("Lägga till");
    public JComboBoxtest(){
     setLayout(new FlowLayout());
     add(new JLabel("Skriv vad som helst"), JLabel.RIGHT);
     add(lista);
     add(ord);
     add(del);
     add(add);
     ord.setHorizontalAlignment(JLabel.RIGHT);
     lista.setEditable(true);
     lista.addActionListener(this);
     add.addActionListener(this);
     del.addActionListener(this);
     setSize(200,100);
     setVisible(true);
     setDefaultCloseOperation(EXIT_ON_CLOSE);
     
 }   
 public void actionPerformed(ActionEvent e){
     String var = (String) lista.getSelectedItem();
     ord.setText("Hi"+ var+"!");
     if(e.getSource()==add)
         lista.addItem(lista);
     if(e.getSource()==del)
         lista.removeItem(lista);
     
 }
 public static void main (String[] args){
     JComboBoxtest box = new JComboBoxtest();
 }   
}

public void actionPerformed(ActionEvent e){
     String var = (String) lista.getSelectedItem();
     ord.setText("Hi"+ var+"!");
     if(e.getSource()==add)
         lista.addItem(var);
     if(e.getSource()==del)
         lista.removeItem(var);
     
 }