Subscribing to an event via non-mono script.

currently i am using the state pattern for AI in my beginner game. here is the steps:

1)i set the initial state of my buildings to idle.

2)idle has two major methods. register and unregister units.

3)register unit is just a counter that counts the number of ai tanks alive.

3)as long as it's less than 5 i delegate to a new state called "building recruiting;which basically recruits till i have 5 units then i go back to idle.

this is the vision i had in mind. how ever here is the problem.

problem:

i get this error (a null value was found when an instance of an object is required).i have been trying to debugg this for last 2-3 hours, i googled the error but not much explanation even on stack overflow.

brief explanation of my scripts:

my logic was that on the start of the TankAI i would fire an event that has the "register unit" method subscribed and the opposite for when it's destroyed.below i will post the BuildingAI script , the building_idle state script and the tank AI script. i am a beginner so i apologize upfront if it is a silly error but i am really lost here. thank you for your time.

TankAI Script

public static event System.Action tankDeath;

 public static event 
     System.Action tankCreation;


 private AIState _myState;
 private string _playerTag;
 private Tank _myTank;

 private void Start() {
     tankCreation ();
     _playerTag = "Player";
     _myTank = GetComponentInChildren<Tank> ();
     myAgent = GetComponentInChildren<NavMeshAgent> ();
     _myState = new Tank_Guard(this,_myTank,myAgent);
 }

private void OnDestroy() {

     tankDeath ();
 }

BuildingAI script

private AIState _myState;

 private Building _myBuilding;

 private void Start () {
     _myBuilding = GetComponent<Building> ();
     _myState = new Building_Idle (_myBuilding);
 }

 private void Update() {
     _myState.updateSate (this);
     //Debug.Log (_myState.UnRegiesterUnit);
 }

 public void SetState (AIState state) {
     _myState = state;
 }

 private void OnEnable() {
     TankAI.tankCreation += _myState.RegiesterUnit;
     TankAI.tankDeath += _myState.UnRegiesterUnit;
 }

 private void OnDisable() {
     TankAI.tankCreation -= _myState.RegiesterUnit;
     TankAI.tankDeath -= _myState.UnRegiesterUnit;
 }

}

and finally the building Idle state script

private int tankCount;

private Building _myBuilding;

public Building_Idle(Building myBuilding) {

     tankCount = 0;

     this._myBuilding = myBuilding;

}

 public override void updateSate (AI myAI) {
     ((BuildingAI)myAI).SetState (this);
     //Debug.Log ("hi");
 }

 public override void RegiesterUnit () {
     tankCount++;
 }

 public override void UnRegiesterUnit (){
     tankCount--;
     Debug.Log (tankCount);
 }