撤消和重做的错误异常

Question

所以我使用堆栈对象执行撤消和重做功能，但当我尝试撤消将朋友添加到数组列表时，我收到此错误：

Exception in thread "main" java.lang.NullPointerException
    at main.main(main.java:256)

这里是主要出现错误的行：

else if("friend".equals(temp.getAction())) //add friend
{
    int newFriendUserIndex = facebook.findUser(temp.getUser2());
    user.defriend(facebook.getUser(newFriendUserIndex));//Error is here

    MyStackObject undo = new MyStackObject(user.toString(), "deFriend", temp.getUser2());
    facebook.redoList.push(undo);
}

MyStackObject类

import java.util.*;
import java.io.Serializable;

public class MyStackObject implements Serializable
{
    private String user1 = null;
    private String action = null;
    private String user2 = null;
    private String like = null;
    private FacebookUser user;

    public MyStackObject(String user1, String action, String user2)
    {
        this.user1 = user1;
        this.action = action;
        this.user2 = user2;
    }

    public MyStackObject(String user1, String like)
    {
        this.user1 = user1;
        this.like = like;
    }

    public MyStackObject(FacebookUser user, String action)
    {
        this.user = user;
        this.user1 = user.toString();
        this.action = action;
    }

    public FacebookUser getUser() 
    {
        return user;
    }

    public void setUser(FacebookUser user) 
    {
        this.user = user;
    }

    public String getUser1() 
    {
        return user1;
    }

    public void setUser1(String user1) 
    {
        this.user1 = user1;
    }

    public String getAction() 
    {
        return action;
    }

    public void setAction(String action) 
    {
        this.action = action;
    }

    public String getUser2() 
    {
        return user2;
    }

    public void setUser2(String user2) 
    {
        this.user2 = user2;
    }

    public String getLike() 
    {
        return like;
    }

    public void setLike(String like) 
    {
        this.like = like;
    }


}

MyStack Class

//EXPLANATION:

public class MyStack implements Serializable
{
    private java.util.ArrayList<MyStackObject> undoList = new java.util.ArrayList<MyStackObject>();

    public MyStack()
    {

    }

      public MyStackObject peek() 
      {
        return undoList.get(getSize() - 1);
      }

      public int getSize() 
      {
        return undoList.size();
      }
      public void push(MyStackObject temp) 
      {
        undoList.add(temp);
      }

      public MyStackObject pop() 
      {
        MyStackObject temp = undoList.get(getSize() - 1);
        undoList.remove(getSize() - 1);
        return temp;
      }

      public boolean isEmpty() 
      {
        return undoList.isEmpty();
      }

}

我发现其他的东西是null，这是撤销选项：

case 12://Undo
                    if(facebook.undoList.isEmpty() && facebook.redoList.isEmpty())
                    {
                        System.out.println("\n\tNo recent actions have been done yet!");
                    }
                    else
                    {
                        MyStackObject temp = null;
                        int secondMenu = undoMenu();
                        String userName;
                        String password;
                        boolean match = false;
                        boolean quit2 = false;
                        Scanner input2 = new Scanner(System.in);
                        FacebookUser user = null;

但我也不确定该改变它。