need help with billboarding

hi! i recently looked up a tutorial because i wanted an object to look at the camera as the player moves around. i used what i saw in the tutorial, but it doesn't work?

here's what i have written:

 public class cameraBillboard : MonoBehaviour
 {
     private void Update()
     {
         transform.LookAt(Camera.main.transform.position, -Vector3.up);
     }
 }
 

and here's the error it gives me:

[21:39:04] NullReferenceException: Object reference not set to an instance of an object cameraBillboard.LateUpdate () (at Assets/cameraBillboard.cs:9)