nullreference on retrieving value on a form

Question

I am currently practicing OOP on C#. I seem to be having a problem on retrieving a string from another form. I have two forms. A publisher form that is where I put the string to be sent to other forms, and a person form which receives the string. Please do refer to the codes below.

public partial class Publisher : Form
{
    Subject s = new Subject();
    Observer p, j;
    Person paul, john;

    public Publisher()
    {
        InitializeComponent();
    }

    private void Publisher_Load(object sender, EventArgs e)
    {


        paul = new Person(s, "Paul", new Observer(s, "Paul"));
        paul.Show(); //opens a new form paul

        john = new Person(s, "John", new Observer(s, "John"));
        john.Show(); //opens a new form john

    }

    private void btnsend_Click_1(object sender, EventArgs e)
    {
        s.Message = textBox1.Text;
        s.Notify();
    }
}

And here is the Person Form.

public partial class Person : Form
{
    string name;
    Subject s;
    Observer p;
    Publisher ps = new Publisher();

    public Person(Subject s, string name, Observer p)
    {
        InitializeComponent();
        this.s = s;
        this.name = name;
        this.p = p;
    }

    private void Paul_Load(object sender, EventArgs e)
    {
        Text = name;
    }

    public void btnregister_Click(object sender, EventArgs e)
    {
        s.Attach(p);
        s.Notify();
        btnregister.Enabled = false;
        btnunregister.Enabled = true;
    }

    public void btnunregister_Click(object sender, EventArgs e)
    {
        s.Detach(p);
        s.Notify();
        btnregister.Enabled = true;
        btnunregister.Enabled = false;
    }

}

This project includes other classes which are implemented to an interface.

public class Observer : IObserver
{
    Subject subj;
    string name;
    Person p;

    public Observer(Subject subj, string name) //Constructor
    {
        this.subj = subj;
        this.name = name;
    }

    public void Update()
    {
        p.textBox1.Text = subj.Message; //I also get nullreference here
    }
}

public interface IObserver
{
    void Update();
}

Here is another class:

public class Subject : ISubject
{
    List<IObserver> observers = new List<IObserver>();

    string message;

    public string Message
    {
        get { return message; }
        set { message = value; }
    }

    public void Attach(IObserver o)
    {
        observers.Add(o);
    }
    public void Detach(IObserver o)
    {
        observers.Remove(o);
    }
    public void Notify()
    {
        foreach (IObserver o in observers)
            o.Update(); //I often get nullreference values from here
    }
}

public interface ISubject
{
    void Attach(IObserver o);
    void Detach(IObserver o);
    void Notify();
}

Here is an output of the forms. Click here.

I believe to have called the right classes, and instantiate the right objects. Although, I can't seem to send a string/message to the other forms. Am I doing it right or am I lacking something on the code?

Here are some objectives for guidance:

1. The publisher form should be able to send messages to the person form/s once they have registered.
2. Once the 'UNREGISTER' button is pressed, they won't be able to receive messages sent from the publisher.

Any help would be deeply appreciated. Thanks!