Duvida Sistema vendas [RESOLVIDO]

Exception in thread "main" java.lang.NullPointerException
	at java.awt.Container.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at Alteraitem.<init>(Alteraitem.java:26)
	at Alteraitem.main(Alteraitem.java:50)

import java.awt.GridLayout;
	import java.awt.event.*;
import javax.swing.*;


	public class Alteraitem extends JFrame {


	JLabel l1, l2, l3, l4;
	JTextField t1, t2, t3, t4 = new JTextField();
	JButton b1, b2;//ok, limpar
	


	public Alteraitem(){ 
		setBounds(200,50,350,85);
		getContentPane().setLayout(new GridLayout(4,5));
		l1 = new JLabel("Código");
		l2 = new JLabel("Descrição");
		l3 = new JLabel("Preço");
		l4 = new JLabel("Quantidade no estoque");
		b1 = new JButton("OK");
		
		b2 = new JButton("Limpar");
		
		getContentPane().add(l1);  getContentPane().add(t1);  
		getContentPane().add(l2);  getContentPane().add(t2);  
		getContentPane().add(l3);  getContentPane().add(t3);  
		getContentPane().add(l4);  getContentPane().add(t4);  
		getContentPane().add(b1);
		  
	}

	
	public static void main(String args[]){
		  JFrame janela = new Alteraitem();
		  janela.setTitle("Cadastro de itens");
		  janela.setVisible(true);
		  janela.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
	}

	}