パネルを表示させるとNullPointerException

前提・実現したいこと

練習も兼ねて、Javaでボタンを押すとページが切り替わるというシステムを作っています。
該当のメソッドでは、現在表示しているパネルをsetVisible(false)、
表示するパネルをsetVisible(true)するという処理のみを行っています。
その、setVisible(true)する際にNullPointerExceptionが発生してしまいます。

発生している問題・エラーメッセージ

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at main.FrameMain.showPanel(FrameMain.java:41)
	at main.TitlePanel.actionPerformed(TitlePanel.java:39)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.Component.processMouseEvent(Component.java:6539)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
	at java.awt.Component.processEvent(Component.java:6304)
	at java.awt.Container.processEvent(Container.java:2239)
	at java.awt.Component.dispatchEventImpl(Component.java:4889)
	at java.awt.Container.dispatchEventImpl(Container.java:2297)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4535)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4476)
	at java.awt.Container.dispatchEventImpl(Container.java:2283)
	at java.awt.Window.dispatchEventImpl(Window.java:2746)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:760)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:84)
	at java.awt.EventQueue$4.run(EventQueue.java:733)
	at java.awt.EventQueue$4.run(EventQueue.java:731)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:730)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
a

該当のソースコード

Java
1package main;
2
3
4import javax.swing.ImageIcon;
5import javax.swing.JButton;
6import javax.swing.JFrame;
7import javax.swing.JPanel;
8
9public class FrameMain extends JFrame
10{
11	public String[] PanelNames = {"title", "difficulty", "question", "result", "option"};
12
13	TitlePanel Ptitle = new TitlePanel(this, PanelNames[0]);
14	DifficultyPanel  Pdifficulty = new DifficultyPanel(this, PanelNames[1]);
15	//TODO QuestionPanel Pquestion = new QuestionPanel(this, PanelNames[2]);
16	//TODO ResultPanel Presult = new ResultPanel(this, PanelNames[3]);
17	//TODO OptionPanel Poption = new OptionPanel(this, PanelNames[4]);
18
19	FrameMain()
20	{
21		this.add(Ptitle);
22		Ptitle.setVisible(true);
23
24		setTitle("Game");
25
26		setSize(400, 400);
27		setLocationRelativeTo(null);
28		setResizable(false);
29	}
30
31	public static void main(String args[])
32	{
33		FrameMain mainFrame = new FrameMain();
34		mainFrame.setDefaultCloseOperation(EXIT_ON_CLOSE);
35		mainFrame.setVisible(true);
36	}
37
38	public static void showPanel(JPanel ShownPanel, JPanel PanelToShow)
39	{
40		ShownPanel.setVisible(false);
41		PanelToShow.setVisible(true);
42	}
43}
44
45class createButton extends JButton
46{
47	public createButton(ImageIcon imagedefault, ImageIcon imageselected)
48	{
49		setIcon(imagedefault);
50		setRolloverIcon(imageselected);
51		setPressedIcon(imageselected);
52		setContentAreaFilled(false);
53		setBorderPainted(false);
54		setFocusable(false);
55	}
56}

Java
1package main;
2
3import java.awt.event.ActionEvent;
4import java.awt.event.ActionListener;
5
6import javax.swing.ImageIcon;
7import javax.swing.JPanel;
8
9public class TitlePanel extends JPanel implements ActionListener
10{
11	FrameMain frameMain;
12	JPanel Pdifficulty;
13
14	TitlePanel(FrameMain fm, String name)
15	{
16		frameMain = fm;
17		this.setName(name);
18
19		ImageIcon start01 = new ImageIcon("./resource/img/start01.png");
20		ImageIcon start02 = new ImageIcon("./resource/img/start02.png");
21		ImageIcon result01 = new ImageIcon("./resource/img/result01.png");
22		ImageIcon result02 = new ImageIcon("./resource/img/result02.png");
23		ImageIcon quit01 = new ImageIcon("./resource/img/quit01.png");
24		ImageIcon quit02 = new ImageIcon("./resource/img/quit02.png");
25
26		createButton B1 = new createButton(start02, start01);
27		createButton B2 = new createButton(result02, result01);
28		createButton B3 = new createButton(quit02, quit01);
29		B1.addActionListener(this);
30
31		this.add(B1);
32		this.add(B2);
33		this.add(B3);
34	}
35
36	@Override
37	public void actionPerformed(ActionEvent e)
38	{
39		FrameMain.showPanel(this, Pdifficulty);
40	}
41}