Why does my program crash when allocate and try to store variable in Vector3 array?

Hi, I am trying to compare two Vector3 which are coordinates to an instantiated object within a margin so that it doesn't instantiate the second object in the same position. But before that I need to store the correct values in an array to compare the current number with in later calculations. But I keep getting either NullReferenceException: Object reference not set to an instance of an object or worse Unity crashes without giving me a bug report. What to do?

my main class:

 public Circle_gen gen;
 
     void Awake () {
         gen = gameObject.GetComponent<Circle_gen> ();
     }
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         gameObject.GetComponent<Circle_gen>();
         if (Input.GetMouseButtonDown (0)) {
             gen.reset ();
             gen.clear ();
             gen.initializePositions ();
             gen.StartCoroutine("generateABC");
         }
     }

my Circle_gen class:

 public GameObject prefabA;
     public GameObject prefabB;
     public GameObject prefabC;
 
     private GameObject aClone;
     private GameObject bClone;
     private GameObject cClone;
 
     private GameObject A;
     private GameObject B;
     private GameObject C;
 
     private int numA;
     private int numB;
     private int numC;
 
     private Vector3 posA, posB, posC;
 
     private bool check = false;
     private float currentX, currentY;
     private int size = 0;
     public Vector3[] positions; //maybe public for text
     private Vector3 position;
 
     private float xMax, xMin, yMax, yMin;
     private float margin = 3.0f;
 
     public void initializePositions () {
         positions = new Vector3[20];
     }
 
     public IEnumerator generateABC(){
         Vector3 center = transform.position;
         numA = 8;
         for (int a = 0; a < numA; a++){
             if (numA != 0) {
                 yield return new WaitForSeconds (1);
             }
             while (check == false) {
                 posA = RandomCircle (center, 3.0f);
                 checkPosition (posA);
             }
             check = false;
             Quaternion rotA = Quaternion.LookRotation(posA-center);
             A = (GameObject)Instantiate(prefabA, posA, rotA);
             A.name = "aCube"+ a;
         }
 
         numB = 8;
         for (int b = 0; b < numB; b++){
             if (numB != 0) {
                 yield return new WaitForSeconds (1);
             }
             while (check == false) {
                 posB = RandomCircle (center, 2.0f);
                 checkPosition (posB);
             }
             check = false;
             Quaternion rotB = Quaternion.LookRotation(posB-center);
             B = (GameObject)Instantiate(prefabB, posB, rotB);
             B.name = "bCube"+ b;
         }
 
         numC = 4;
         for (int c = 0; c < numC; c++){
             if (numC != 0) {
                 yield return new WaitForSeconds (1);
             }
             while (check == false) {
                 posC = RandomCircle (center, 1.0f);
                 checkPosition (posB);
             }
             check = false;
             Quaternion rotC = Quaternion.LookRotation(posC-center);
             C = (GameObject)Instantiate(prefabC, posC, rotC);
             C.name = "cCube"+ c;
         }
     }
 
     private Vector3 RandomCircle ( Vector3 center ,   float radius  ){
         Vector3 pos;
         float ang = Random.value * 360;
         pos.x = center.x + radius * Mathf.Sin (ang * Mathf.Deg2Rad);
         pos.y = center.y;
         pos.z = center.z + radius * Mathf.Cos (ang * Mathf.Deg2Rad);
         return pos;
     }
 
     private void checkPosition (Vector3 currentPos) {
         if (positions != null) {
             currentX = currentPos.x;
             currentY = currentPos.y;
             for (int i = 0; i < positions.Length; i++) {
                 position = positions [i];
                 xMax = position.x + margin;
                 xMin = position.x - margin;
                 yMax = position.y + margin;
                 yMin = position.y - margin;
                 if (currentX > xMax && currentX < xMin && currentY > yMax && currentY < yMin) {
                     print ("ok");
                     size++;
                     positions [size] = currentPos;
                     check = true;
                 } else {
                     print ("not ok");
                 }
             }
         } else {
             positions [0] = currentPos;
             check = true;
         }
         return;
     }
 
     public void clear (){
         if (positions != null){
             positions = null;
         }
     }
 
     public void reset(){
 
         for (int a = 0; a < numA; a++) {
             aClone = GameObject.Find ("aCube" + a);
             if (aClone != null) {
                 Destroy (aClone);
             }
         }
         for (int b = 0; b < numB; b++) {
             bClone = GameObject.Find ("bCube" + b);
             if (bClone != null) {
                 Destroy (bClone);
             }
         }
         for (int c = 0; c < numC; c++) {
             cClone = GameObject.Find ("cCube" + c);
             if (cClone != null) {
                 Destroy (cClone);
             }
         }
     }

Thank you for any help you could give me.