Null Reference Raycast

     public class RaycastExample : MonoBehaviour {
      
         void Update () {
            if (Input.GetMouseButtonDown (0))
            {
              Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
              RaycastHit hit;
              if (Physics.Raycast(ray, out hit))
              {
               Debug.Log ("Name = " + hit.collider.name);
               Debug.Log ("Tag = " + hit.collider.tag);
               Debug.Log ("Hit Point = " + hit.point);
               Debug.Log ("Normal = " + hit.normal);
               Debug.Log ("Object position = " + hit.collider.gameObject.transform.position);
               Rigidbody rb = hit.collider.rigidbody;
      
               if (rb != null)
                   Debug.Log ("Velocity = " + rb.velocity);
               Debug.Log ("--------------");
              }
            }
         }
     }