Getting error NullReferenceException on instantiate

T$$anonymous$$s morning all worked so fine and know when i wanted to continue my project i'm getting an error. I work with collab and i got an error earlier and needed to chage turret to Turret to can update the project to collab. P.S. Script node.cs color the hover mouse node only if you clicked a button before that. That worked in morning but now i the hover color of node appear without you to click that button. I don't understand. I'm mind f**k... I'm getting t$$anonymous$$s error:

 UnityEngine.Object.Internal_InstantiateSingle (UnityEngine.Object data, Vector3 pos, Quaternion rot) (at C:/buildslave/unity/build/artifacts/generated/common/runtime/UnityEngineObjectBindings.gen.cs:51)
 UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:157)
 UnityEngine.Object.Instantiate[GameObject] (UnityEngine.GameObject original, Vector3 position, Quaternion rotation) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:207)
 BuildManager.BuildTurretOn (.Node node) (at Assets/Scripts/BuildManager.cs:29)
 Node.OnMouseDown () (at Assets/Scripts/Node.cs:35)
 UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32)

Node.cs using UnityEngine; using UnityEngine.EventSystems;

 public class Node : MonoBehaviour {
     [Header("Optional")]
     public GameObject turret;
     public Color hoverColor;
     public Vector3 positionOffset;
     private Renderer rend;
     private Color startColor;
     BuildManager buildManager;
 
     void Start()
     {
         rend = GetComponent<Renderer> ();
         startColor = rend.material.color;
         buildManager = BuildManager.instance;
     }
 
     public Vector3 GetBuildPosition()
     {
         return transform.position + positionOffset;
     }
 
     void OnMouseDown()
     {
         if (EventSystem.current.IsPointerOverGameObject ())
             return;
         if (!buildManager.CanBuild)
             return;
         if (turret != null) {
             Debug.Log ("Ai creat deja o arma aici!");
             return;
         }
         buildManager.BuildTurretOn (t$$anonymous$$s);
     }
 
     void OnMouseEnter()
     {
         if (EventSystem.current.IsPointerOverGameObject ())
             return;
         if (!buildManager.CanBuild)
             return;
         rend.material.color = hoverColor;
     }
 
     void OnMouseExit()
     {
         rend.material.color = startColor;
     }
 }

BuildManager.cs

 using UnityEngine;
 
 public class BuildManager : MonoBehaviour {
 
     public static BuildManager instance;
 
     void Awake()
     {
         if (instance != null) {
             Debug.Log ("More than one BuildManager in scene!");
             return;
         }
         instance = t$$anonymous$$s;
     }
     public GameObject standardTurretPrefab;
     public GameObject anotherTurretPrefab;
 
     public TurretBluePrint turretToBuild;
 
     public bool CanBuild { get { return turretToBuild != null; } }
 
     public void BuildTurretOn(Node node)
     {
         if (PlayerStats.Money < turretToBuild.cost) {
             Debug.Log ("Nu ai bani destui!");
             return;
         }
         PlayerStats.Money -= turretToBuild.cost;
         GameObject turret = (GameObject)Instantiate (turretToBuild.prefab, node.GetBuildPosition(), Quaternion.identity);
         node.turret = turret;
         Debug.Log("Turret build! Money left: " + PlayerStats.Money);
     }
 
     public void SelectTurretToBuild(TurretBluePrint turret)
     {
         turretToBuild = turret;
     }
 }

TurretBluePrint.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [System.Serializable]
 public class TurretBluePrint{
 
     public GameObject prefab;
     public int cost;
 
 }

The errors from scripts:

BuildManager.cs Line 29:

 GameObject turret = (GameObject)Instantiate (turretToBuild.prefab, node.GetBuildPosition(), Quaternion.identity);

Node.cs Line 35:

 buildManager.BuildTurretOn (t$$anonymous$$s);