System. NullReferenceException c#?

Tanulgatom az OOP-t, de beleütköztem a fenti hibába, és öteletem sincs, mit csináltam rosszul.

Nagyra értékelnék minden segítséget!

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace ConsoleApp6

{

class Program

{

public class Owner

{

int age;

string name;

public string Name { get; set; }

public string Age { get; set; }

public Owner(int age,string name)

{

this.age = age;

this.name = name;

}

}

public class Slave

{

Owner o;

int age;

string name;

public string Name { get; set; }

public string Age { get; set; }

public Owner O{ get; set; }

public Slave(int age, string name, Owner o)

{

this.age = age;

this.name = name;

this.o = o;

}

}

public class Gun

{

Slave user;

int age;

string name;

public string Name { get; set; }

public string Age { get; set; }

public Slave User{ get; set; }

public Gun(int age, string name,Slave user)

{

this.age = age;

this.name = name;

this.user = user;

}

}

static void Main(string[] args)

{

Gun[] w = new Gun[2]

{

new Gun(1,"a",new Slave(2,"b", new Owner(3,"c"))),

new Gun(4,"d",new Slave(5,"e", new Owner(6,"f")))

};

Console.WriteLine(w[0].User.O.Age);

Console.ReadLine();

}

}

}