Unhandled Exception: Access violation reading location

Unhandled exception at 0x001AF3B5 in test.exe: 0xC0000005: Access violation reading location 0x00000000.

getting an unhandled exception when i implement this code in my project http://sourcemaking.com/design_patterns/command/cpp/2

#include <iostream>  
#include <string>  
using namespace std;
class Person;

class Command
{
// 1. Create a class that encapsulates an object and a member function
// a pointer to a member function (the attribute's name is "method")
Person *object; //    
void(Person:: *method)();
  public:
    Command(Person *obj = 0, void(Person:: *meth)() = 0)
{
    object = obj; // the argument's name is "meth"
    method = meth;
}
void execute()
{
    (object-> *method)(); // invoke the method on the object
}
};

class Person
{
string name;

// cmd is a "black box", it is a method invocation
// promoted to "full object status"
Command cmd; 
  public:
Person(string n, Command c): cmd(c)
{
    name = n;
}
void talk()
{
    // "this" is the sender, cmd has the receiver
    cout << name << " is talking" << endl;
    cmd.execute(); // ask the "black box" to callback the receiver
}
void passOn()
{
    cout << name << " is passing on" << endl;

    // 4. When the sender is ready to callback to the receiver,
    // it calls execute()
    cmd.execute(); 
}
void gossip()
{
    cout << name << " is gossiping" << endl;
    cmd.execute();
}
void listen()
{
    cout << name << " is listening" << endl;
}
};

int main()
{
  // Fred will "execute" Barney which will result in a call to passOn()
  // Barney will "execute" Betty which will result in a call to gossip()
  // Betty will "execute" Wilma which will result in a call to listen()
  Person wilma("Wilma", Command());
  // 2. Instantiate an object for each "callback"
  // 3. Pass each object to its future "sender"
  Person betty("Betty", Command(&wilma, &Person::listen));
  Person barney("Barney", Command(&betty, &Person::gossip));
  Person fred("Fred", Command(&barney, &Person::passOn));
  fred.talk();
}

Not sure at all why? I implemented it as it no changes except the output is different and only 2 people instead of the 4. Tried a bit of debugging and its erroring at execute() in command.

Any help or tips would be great!

EDIT: Actually after some testing its when i change the people and the calls that it is happening not sure why.

    Person player("Player", Command());
    Person master("House Master", Command(&player, &Person::passOn));
    master.talk();

The problem is that you allow empty command, and you even execute it without checking.

Changing the Command's constructor to this should solve the problem :

Command(Person &obj, void(Person:: *meth)())