using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class creatingobjectslist : MonoBehaviour {

    public GameObject TargetPrefab;

    void Start () {
        for (int i = 0; i < 5; i++){
            Instantiate(TargetPrefab, new Vector3(i * 1.5f, 0f, 0f), Quaternion.identity);
            TargetPrefab.name = "Ben" +i;
        }
    }
}

using UnityEngine;
using System.Collections;
public class MoveCubeTogrid : MonoBehaviour {
    UnityEngine.AI.NavMeshAgent agent;

    void Start () {
        agent = GetComponent< UnityEngine.AI.NavMeshAgent >();
    }

    void Update () {
        if (Input.GetMouseButtonDown(0)) {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;

            if (Physics.Raycast(ray, out hit, Mathf.Infinity)) {
                    //agent.destination = hit.point;
                    agent.destination = GameObject.Find("Ben0").transform.position;
            }
        }
    }
}

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

TargetPrefab.name = "Ben" +i;