"NullReferenceException, object reference not set to an instance of an object"

Hello everyone! Well i m new at Unity and i m trying to learn the c# and create a small game just to see how it goes! i m using the Brakeys tutorial on how to make an TD game. After a deep search in all the searching machines and writting the script from the begging several times (and i believe its right) i get an "NullReferenceException, object reference not set to an instance of an object" while i m on the play mode and trying to build a tower (on a "buildplace") for which i have made a button. when i push the icon i get the Debug.Log i want (tower selected) but i also get this error so i cant procceed.

using UnityEngine;

public class Shop : MonoBehaviour {

 BuildManager buildManager;

 void Start () {
     buildManager = BuildManager.Instance;
 }

 public void PurchaseTurret5() {
     Debug.Log ("Turret 5 Selected");
     buildManager.SetTurretToBuild (buildManager.Turret5Prefab); // ERROR
 }

 public void PurchaseAnotherTurret() {
     Debug.Log ("Another Turret Selected");
     buildManager.SetTurretToBuild (buildManager.AnotherTurretPrefab);
 }

}

This is my script and i get the error on 13th line. Do anyone see anything wrong up there?? (I also made a Buildmanager script, i dont know if there is something i did wrong back there, cause i guess those 2 scripts are now communicate) Any help will be appriciated! Thanks a lot !