Problemas com lookAndFeel

package view;

import java.awt.Color;
import java.awt.Font;
import java.io.ObjectInputStream.GetField;

import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.UIManager;
import javax.swing.ViewportLayout;

public class VIEWUtensiliValori extends JFrame {

	private static final long serialVersionUID = 1L;

	private static VIEWMenuFerramentas mn_ferramentas = new VIEWMenuFerramentas();
	private static VIEWCabecalho jp_cabecalho = new VIEWCabecalho();
	private static VIEWBarraMenu bm_menu = new VIEWBarraMenu();
	
	private static JPanel jp_conteudo = new VIEWOrdemObjetiva();

	public VIEWUtensiliValori() {
		setSize(500, 500);
		setLocationRelativeTo(null);
		getContentPane().setBackground(new Color(216, 216, 252));
		setLayout(null);		
		setTitle("Utensili Valori");
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		addMembros();		
		
		try {
			UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");  
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
		
		setVisible(true);
	}

	private void addMembros() {
		// Seta posicao do menu de ferramentas
		mn_ferramentas.setBounds(0, 55, getWidth(), 30);

		jp_cabecalho.setBounds(5, 0, 475, 45);
		jp_conteudo.setBounds(5, 90, getWidth() - 27, 200);

		setJMenuBar(bm_menu);
		add(jp_cabecalho);
		add(mn_ferramentas);
		add(jp_conteudo);		
	}

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

	public static JPanel getJp_conteudo() {
		return jp_conteudo;
	}	

}