Error : Animator has not been initialized. UnityEngine.Animator:SetBool(String, Boolean)

I notice my animation on my enemy ai wasn't working . It only idles . I look at the console and I saw this error : Animator has not been initialized.

UnityEngine.Animator:SetBool(String, Boolean)

Enemyai:Update() (at Assets/Enemyai.cs:40)

I need to change something on line 40 . I used this same script on a another enemy ai . Here is my script :

  using UnityEngine;
  using System.Collections;
  
  public class Enemyai : MonoBehaviour {
  public Transform player;
  static Animator anim;
      void Start () 
      {
              anim = GetComponent<Animator> ();
      }
      
  
      void Update () 
      {
          if (Vector3.Distance(player.position, this.transform.position) < 10)
          {
              Vector3 direction = player.position - this.transform.position;
              direction.y = 0;
              
              this.transform.rotation = Quaternion.Slerp (this.transform.rotation,Quaternion.LookRotation(direction), 0.1f);
              
              anim.SetBool("isIdle",false);
              if(direction.magnitude > 2.6 )
              {
                  this.transform.Translate(0,0,0.05f);
                  anim.SetBool("isMoving",true);
                  anim.SetBool("isAttacking",false);
                  }
              else
              {
                  anim.SetBool("isAttacking",true);
                  anim.SetBool("isMoving",false);
                  
              }
          }
         else
         {
             anim.SetBool("isIdle",true);
             anim.SetBool("isMoving",false);
             anim.SetBool("isAttacking",false);
             
         }
      
      
      }
  }