NullReferenceExeption

Hey guys. I found this script in a older tutorial from 2015 and I cannot fix it. It gives me "NullReferenceExeption: Object reference not set to an instance of an object Spawner.OnEnable () (at Assets/Scripts/Spawner.cs:14) "

and

"NullReferenceExeption: Object reference not set to an instance of an object Spawner.OnDisable () (at Assets/Scripts/Spawner.cs:14

at the beginning and end of the consol.

{

public GameObject objectToSpawn;

public int numberOfEnemies;

private float spawnRadius = 30;

private Vector3 spawnPosition;

private GameManager_EventMaster eventMasterScript;

 void OnEnable()
 {
     SetInitialReferences();
     eventMasterScript.myGeneralEvent += SpawnObject;
 }

 void OnDisable()
 {
     eventMasterScript.myGeneralEvent -= SpawnObject;
 }

     
 void SpawnObject()
 {
     for (int i = 0; i < numberOfEnemies; i++)
     {
         spawnPosition = transform.position + Random.insideUnitSphere * spawnRadius;
         Instantiate (objectToSpawn,spawnPosition,Quaternion.identity);
     }
 }
 void SetInitialReferences()
 {
     eventMasterScript = GameObject.Find ("GameManager").GetComponent<GameManager_EventMaster>();
 }

}

And the GameManager_EventMaster:

{

public delegate void GeneralEvent ();

public event GeneralEvent myGeneralEvent;

 public void CallMyGeneralEvent()
 {
     if (myGeneralEvent != null) 
     {
         myGeneralEvent ();    
     }
 }

}

Tutorial: https://www.youtube.com/watch?v=e3OimELNyMc∈dex=12&list=PLwyZdDTyvucwjwqucleVQB7U12H2JPvg5