NavMeshAgent.CalculatePath keeps getting null reference exception

NullReferenceException: Object reference not set to an instance of an object UnityEngine.AI.NavMeshAgent.CalculatePath (UnityEngine.Vector3 targetPosition, UnityEngine.AI.NavMeshPath path) (at < 030353032b914045a0b96bea329b98d3 >:0)

the function that uses calculate path (agent is NavMeshAgent that uses this function, CalculatePathCost from here https://forum.unity.com/threads/cost-of-a-navmeshpath.437486/)

 public Dictionary PathEvaluation(Transform[] this_targets)
     {
         Dictionary<Vector3, float> pathsdic = new Dictionary<Vector3, float>();
         NavMeshPath path = new NavMeshPath();
         foreach (Transform target in this_targets)
         {
             agent.CalculatePath(target.position, path);
             if (path.status.Equals(NavMeshPathStatus.PathComplete))
             {
                 pathsdic.Add(target.position, CalculatePathCost(path));
             }
         }
         return pathsdic;
     }