"Object at index 0 is null"

Hi, I'm making a tower defense, and when i want to draw the line to the enemy, I ve this eror : "Object at index 0 is null". The fact is, my program is working, but I've this error when i launch it. Thanks for the answer

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Turret : MonoBehaviour {
 
     [Header("Finding the closest enemy")]
     public Transform target;
 
     [Header("Values")]
     public float range = 15f;
     public string enemyTag = "Enemy";
 
     [Header("LineRenderer")]
     private LineRenderer line;
     public Transform origin;
     public float lineDrawSpeed = 1f;
     private float counter;
     private float dist;
 
     void Start () {
         line = GetComponent<LineRenderer>();
         line.SetPosition(0, origin.position);
         line.SetWidth(0.45f, 0.45f);
 
         InvokeRepeating("UpdateTarget", 0f, 0.2f);
     }
 
     void UpdateTarget()
     {
         GameObject[] enemies = GameObject.FindGameObjectsWithTag(enemyTag);
         float shortestDistance = Mathf.Infinity;
         GameObject nearestEnemy = null;
 
         foreach(GameObject enemy in enemies)
         {
             float distanceToEnemy = Vector3.Distance(transform.position, enemy.transform.position);
             if(distanceToEnemy < shortestDistance)
             {
                 shortestDistance = distanceToEnemy;
                 nearestEnemy = enemy;
             }
         }
 
         if(nearestEnemy !=null && shortestDistance <= range)
         {
             target = nearestEnemy.transform;
         }
         else
         {
             target = null;
         }
 
 
     }
     
     void Update () {
         if (target == null)
         {
             
             counter = dist = 0;
             line.enabled = false;
         }
         else
         {
             line.enabled = true;
             dist = Vector3.Distance(origin.position, target.position);
             Vector3 pointA = origin.position;
             Vector3 pointB = target.position;
             if (counter < dist)
             {
                 counter += 0.1f / lineDrawSpeed;
             }
             float l = Mathf.Lerp(0, dist, counter);
             Vector3 pointAlongLine = l * Vector3.Normalize(pointB - pointA) + pointA;
             line.SetPosition(1, pointAlongLine);
 
         }
 
     }
     void OnDrawGizmosSelected()
     {
         Gizmos.color = Color.red;
         Gizmos.DrawWireSphere(transform.position, range);
     }
 }


 ArgumentException: Object at index 0 is null
 UnityEditor.SerializedObject..ctor (UnityEngine.Object[] objs, UnityEngine.Object context) (at C:/buildslave/unity/build/Editor/Mono/SerializedObject.bindings.cs:39)
 UnityEditor.Editor.GetSerializedObjectInternal () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:553)
 UnityEditor.Editor.get_serializedObject () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:452)
 UnityEditor.UIElements.InspectorElement..ctor (UnityEditor.Editor editor, Mode mode) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:133)
 UnityEditor.UIElements.EditorElement.Init () (at C:/buildslave/unity/build/Editor/Mono/Inspector/EditorElement.cs:84)
 UnityEditor.UIElements.EditorElement..ctor (Int32 editorIndex, UnityEditor.InspectorWindow iw) (at C:/buildslave/unity/build/Editor/Mono/Inspector/EditorElement.cs:62)
 UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1392)
 UnityEditor.InspectorWindow.RebuildContentsContainers () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:639)
 UnityEditor.InspectorWindow.RedrawFromNative () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:311)