NullReferenceException: Object reference not set to an instance of an object

I keep getting an error stating, NullReferenceException: Object reference not set to an instance of an object opendoor.Update () (at Assets/opendoor.cs:26). I'm using this for my door to open and close it. Can someone tell me what's wrong with this?

 using UnityEngine;
 using System.Collections;
 
 public class opendoor : MonoBehaviour {
 public float doorOpenAngle = 90.0f;
 public float doorCloseAngle = 0.0f;
 public float doorAnimSpeed = 2.0f;
 private Quaternion doorOpen = Quaternion.identity;
 private Quaternion doorClose = Quaternion.identity;
 private Transform playerTrans = null;
 public bool doorStatus = false; //false is close, true is open
 private bool doorGo = false; //for Coroutine, when start only one
 void Start() {
     doorStatus = false; //door is open, maybe change
     //Initialization your quaternions
     doorOpen = Quaternion.Euler (0, doorOpenAngle, 0);
     doorClose = Quaternion.Euler (0, doorCloseAngle, 0);
     //Find only one time your player and get him reference
     playerTrans = GameObject.Find ("Player").transform;
 }
 void Update() {
     //If press F key on keyboard
     if (Input.GetKeyDown(KeyCode.F) && !doorGo) {
         //Calculate distance between player and door
         if (Vector3.Distance(playerTrans.position, this.transform.position) < 5f) {
             if (doorStatus) { //close door
                 StartCoroutine(this.moveDoor(doorClose));
             } else { //open door
                 StartCoroutine(this.moveDoor(doorOpen));
             }
         }
     }
 }
 public IEnumerator moveDoor (Quaternion dest)
 {
     doorGo = true;
     //Check if close/open, if angle less 4 degree, or use another value more 0
     while (Quaternion.Angle (transform.localRotation, dest) > 4.0f) {
         transform.localRotation = Quaternion.Slerp (transform.localRotation, dest, Time.deltaTime * doorAnimSpeed);
         //UPDATE 1: add yield
         yield return null;
     }
     //Change door status
     doorStatus = !doorStatus;
     doorGo = false;
     //UPDATE 1: add yield
     yield return null;
 }

}