When I try to run my script I get this error:

NullReferenceException Movementscript.FixedUpdate () (at Assets/Movementscript.js:7)

Here is my script:

   var speed=20.0;
    function FixedUpdate () {

    var torque=Vector3(Input.GetAxisRaw("Vertical"), 0, -Input.GetAxisRaw("Horizontal"));
    if(torque.magnitude > 0.0){
       torque=Camera.main.transform.TransformDirection(torque);
       rigidbody.angularVelocity=Vector3.Lerp(rigidbody.angularVelocity, torque * 2, 0.3);
    }

if (Input.GetButton("W"))
{

rigidbody.AddRelativeTorque (2, 0, 0);

rigidbody.AddRelativeForce (2, 0, 0);
}


if (Input.GetButton("S"))
{

rigidbody.AddRelativeTorque (-2, 0, 0);
rigidbody.AddRelativeForce (-2, 0, 0);
}

if (Input.GetButton("A"))
{
rigidbody.AddRelativeTorque (0, 0, -2);
rigidbody.AddRelativeForce (0, 0, -2);
}


if (Input.GetButton("D"))
{
rigidbody.AddRelativeTorque (0, 0, 2);
rigidbody.AddRelativeForce (0, 0, 2);
}
}

I don't see any problem with the script, so can someone explain to me what's wrong?