Java SE (Java Platform, Standard Edition)

Hi!

I have a following code, which should add p1 panel to a tabbed pane and then add this pane to a frame. Everything seems to be correct, but it just doesn't add a panel to a pane and only adds a pane to a frame.

import java.awt.BorderLayout;
import java.awt.Dimension;
import javax.swing.Icon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;
import javax.swing.JTextField;
import net.miginfocom.swing.MigLayout;

public class ProtoForm extends JFrame
	{
	private JTabbedPane tpCategories;
	private JLabel l1, l2;
	private JTextField tf1, tf2;
	private JPanel p1;
	
	public ProtoForm()
		{
		super("ProtoForm");				
		setSize(new Dimension(500, 375));			
		
		tpCategories = new JTabbedPane();
		addSoftComponents(p1);
		addTab(tpCategories, "Soft", null, p1, null);
		
		add(tpCategories);
		}
	
	private void addTab(JTabbedPane tp, String title, Icon icon, JPanel panel, String tip)
		{
		tp.addTab(title, icon, panel, tip);
		}
	
	private void addSoftComponents(JPanel panel)
		{
		MigLayout layout = new MigLayout();
		panel = new JPanel();
		panel.setLayout(layout);
		
		JLabel l1 = new JLabel("label 1");
		JLabel l2 = new JLabel("label 2");
		JTextField tf1 = new JTextField(35);
		JTextField tf2 = new JTextField(35);
		
		panel.add(l1);
		panel.add(tf1, "wrap");
		panel.add(l2);
		panel.add(tf2, "wrap");
		}
	}

But everything works correctly if I add p1 = new JPanel(); to the the main(). Here is the working code (!!!! marks modified parts of the code):

import java.awt.BorderLayout;
import java.awt.Dimension;
import javax.swing.Icon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;
import javax.swing.JTextField;
import net.miginfocom.swing.MigLayout;


public class ProtoForm extends JFrame
	{
	private JTabbedPane tpCategories;
	private JLabel l1, l2;
	private JTextField tf1, tf2;
	private JPanel p1;
	
	public ProtoForm()
		{
		super("ProtoForm");				
		setSize(new Dimension(500, 375));			
		
		tpCategories = new JTabbedPane();
!!!!           	 p1 = new JPanel();
		addSoftComponents(p1);
		addTab(tpCategories, "Soft", null, p1, null);
		
		add(tpCategories);
		}
	
	private void addTab(JTabbedPane tp, String title, Icon icon, JPanel panel, String tip)
		{
		tp.addTab(title, icon, panel, tip);
		}
	
	private void addSoftComponents(JPanel panel)
		{
		MigLayout layout = new MigLayout();
!!!!		 //panel = new JPanel();
		panel.setLayout(layout);
		
		JLabel l1 = new JLabel("label 1");
		JLabel l2 = new JLabel("label 2");
		JTextField tf1 = new JTextField(35);
		JTextField tf2 = new JTextField(35);
		
		panel.add(l1);
		panel.add(tf1, "wrap");
		panel.add(l2);
		panel.add(tf2, "wrap");
		}
	}

Can someone please explain to me why is it so and how should I redesign the code to create and add panels in procedures outside of main()?
In case someone might need MigLayout it is from http://www.miglayout.com/.