Object reference not set to an instance of an object.

The following code throws this error:

An unhandled exception of type 'System.NullReferenceException' occurred in Classes.exe
Additional information: Object reference not set to an instance of an object.

When I call the form load function it throws the above error and Im not sure why.

namespace Classes
{
///

public class ActSessions

public ActSessions()

//

// TODO: Add constructor logic here

//

public class User

public string name;

public string tabindex;

public User [] UserArray = new User[100]; 

private void Form1_Load(object sender, System.EventArgs e)

ActSessions Active = new ActSessions();