求大神帮我看看这个贪吃蛇的空指针错误（半成品）

[weixin_36992079]

代码如下，写到一半运行一直是这个错误，写不下去了，查了两小时也没发现错在哪儿，一直跟着视频学习的，没发现也视频里代码有什么出入。
Yard

import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class Yard extends Frame {


@Override
public void update(Graphics g) {
if(offScreenImage == null) {
offScreenImage = this.createImage(COLS * BLOCK_SIZE,ROWS * BLOCK_SIZE);

}
Graphics gOff = offScreenImage.getGraphics();
paint(gOff);
g.drawImage(offScreenImage,0,0, null);
}

public static final int ROWS = 50;
public static final int COLS = 50;
public static final int BLOCK_SIZE = 10;

Snake s = new Snake();

Image offScreenImage = null;

public void paint(Graphics g) {
Color c = g.getColor();
g.setColor(Color.GREEN);
g.fillRect(0,0,COLS * BLOCK_SIZE,ROWS * BLOCK_SIZE);
g.setColor(Color.RED);
//画横线
for(int i=1;i<ROWS;i++){
g.drawLine(0, BLOCK_SIZE*i, COLS*BLOCK_SIZE, BLOCK_SIZE*i);
}
//画竖线
for(int i=1;i<COLS;i++){
g.drawLine(BLOCK_SIZE*i,0, BLOCK_SIZE*i,ROWS*BLOCK_SIZE);
}
g.setColor(c);
s.draw(g);
}



public class PaintThread implements Runnable {
public void run() {
while (true) {
repaint();
try {
Thread.sleep(50);
}catch(InterruptedException e) {
e.printStackTrace();
}
}
}
}
//Image

public void launch() {
this.setLocation(200,200);
this.setSize(ROWS * BLOCK_SIZE,COLS * BLOCK_SIZE);
this.setVisible(true);
this.addKeyListener(new KeyMonitor());

new Thread(new PaintThread()).start();

this.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}

public static void main(String[] args) {
new Yard().launch();

}

private class KeyMonitor extends KeyAdapter {

@Override
public void keyPressed(KeyEvent e) {

s.keyPressed(e);
}

}

}

snake

import java.awt.color.*;
import java.awt.event.KeyEvent;
import java.awt.Color;
import java.awt.Graphics;


public class Snake {
private Node head = null;
private Node tail = null;
private int size = 0;

private Node n = new Node(20,20,Dir.L);

public Snake() {
head = n;
tail = n;
size = 1;
}

public void addToTail() {
Node node = null;
switch(tail.dir) {
case L:
node = new Node(tail.row,tail.col + 1,tail.dir);
break;
case R:
node = new Node(tail.row,tail.col - 1,tail.dir);
break;
case U:
node = new Node(tail.row + 1,tail.col,tail.dir);
break;
case D:
node = new Node(tail.row - 1,tail.col,tail.dir);
break;

}
tail.next = node;
node.prev = tail;
tail = node;
size ++;

}

public void addToHead() {
Node node = null;
switch(head.dir) {
case L:
node = new Node(head.row,head.col - 1,head.dir);
break;
case R:
node = new Node(head.row,head.col + 1,head.dir);
break;
case U:
node = new Node(head.row - 1,head.col,head.dir);
break;
case D:
node = new Node(head.row + 1,head.col,head.dir);
break;
}
node.next = head;
head.prev = node;
node = head;
size ++;
}

public void draw(Graphics g) {
if(size<=0) return;

for(Node n=head;n != null;n= n.next) {
n.draw(g);
}
move();
}

private void move() {
addToHead();
deleteFromTail();

}

private void deleteFromTail() {
if(size == 0) return;
tail = tail.prev;
tail.next = null;

}



private class Node {
int w = Yard.BLOCK_SIZE;
int h = Yard.BLOCK_SIZE;
int row,col;
Dir dir = Dir.L;
Node next = null;
Node prev = null;

Node(int row,int col,Dir dir) {
this.row = row;
this.col = col;
this.dir = dir;

}

void draw(Graphics g) {
Color c = g.getColor();
g.setColor(Color.BLACK);
g.fillRect(Yard.BLOCK_SIZE * col,Yard.BLOCK_SIZE * row,w,h);
g.setColor(c);
}
}



public void keyPressed(KeyEvent e) {
int key = e.getKeyCode();
switch(key) {
case KeyEvent.VK_LEFT:
head.dir = Dir.L;
break;
case KeyEvent.VK_RIGHT:
head.dir = Dir.R;
break;
case KeyEvent.VK_UP:
head.dir = Dir.U;
break;
case KeyEvent.VK_DOWN:
head.dir = Dir.D;
break;

}

}
}

Dir


public enum Dir {
L, U, R, D
}

[___d]

能说哪报错不啊。。。贴一堆代码