NullReferenceException Error

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?