GameObject is not activating from StateMachineBehavior script

I'm trying to reference "Hitbox" GameObject that's in the scene and activate it when the animation begins. I run into this error though, "Object reference not set to an instance of an object." I'm not sure why this happens and I have no idea how to fix it. Please help!

 private GameObject hitbox;
 private GameObject hand;

 // OnStateEnter is called when a transition starts and the state machine starts to evaluate this state
 override public void OnStateEnter(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 {
     hitbox = GameObject.Find("Hitbox");
     hand = GameObject.Find("LeftHand");
     hitbox.SetActive(true);
 }

 // OnStateUpdate is called on each Update frame between OnStateEnter and OnStateExit callbacks
 override public void OnStateUpdate(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 {
     hitbox.transform.localScale = new Vector3(1, 1, 1.2f);
     hitbox.transform.position = new Vector3(hand.transform.position.x, hand.transform.position.y, hand.transform.position.z);
 }

 // OnStateExit is called when a transition ends and the state machine finishes evaluating this state
 override public void OnStateExit(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 {
     hitbox.SetActive(false);
 }

 // OnStateMove is called right after Animator.OnAnimatorMove()
 //override public void OnStateMove(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 //{
 //    // Implement code that processes and affects root motion
 //}

 // OnStateIK is called right after Animator.OnAnimatorIK()
 //override public void OnStateIK(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 //{
 //    // Implement code that sets up animation IK (inverse kinematics)
 //}