Zombie AI chasing

hello, i'm using script from youtube videos https://www.youtube.com/watch?v=gXpi1czz5NA, and the script is: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class EnemyAI : MonoBehaviour {

 public Transform player;
 public Animator anim;
 public float moveSpeed = 3f;
 public int sightRange = 20;
 public int minDist = 2;

 // Use this for initialization
 void Start () {
     anim = GetComponent<Animator>();
 }
 
 // Update is called once per frame
 void Update () {
     transform.LookAt(player);

     var direction = player.position - transform.position;
     var angle = Vector3.Angle(direction, this.transform.forward);
     if (angle < 180 && direction.magnitude < sightRange)
     {
         var rotation = Quaternion.Slerp(
         this.transform.rotation, Quaternion.LookRotation(direction), Time.deltaTime);
         rotation.x = 0;
         rotation.z = 0;
         transform.rotation = rotation;
         anim.SetBool("walk", true);

         if (direction.magnitude > minDist)
         {
             transform.Translate(0, 0, Time.deltaTime * moveSpeed);
             DisableAnim();
         }
     }
 }

 void DisableAnim()
 {
     anim.SetBool("walk", false);
 }

}

why i always get an error like:

KeyNotFoundException: The given key was not present in the dictionary. System.Collections.Generic.Dictionary`2[System.String,UnityEditor.Graphs.AnimationStateMachine.EdgeInfo].get_Item (System.String key) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150) UnityEditor.Graphs.AnimationStateMachine.Graph.GetEdgeInfo (UnityEditor.Graphs.Edge edge) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/Graph.cs:282) UnityEditor.Graphs.AnimationStateMachine.GraphGUI.SyncGraphToUnitySelection (Boolean force) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/GraphGUI.cs:286) UnityEditor.Graphs.AnimatorControllerTool.DetectAnimatorControllerFromSelection () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:392) UnityEditor.Graphs.AnimatorControllerTool.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:402)

pls help me, Thanks b4