NullReferenceException error in an array of objects

I can't understand why this post was closed for "duplicate question" .-. I don't see the other question, where is it?

Hi, I have a problem with a script.

 [...]
 void CleanCubes (){
             GameObject[] allCube = GameObject.FindGameObjectsWithTag("Cube");
             foreach(GameObject current in allCube)
             {
                 foreach(GameObject current2 in allCube)
                 {
                     if(current.transform.position == current2.transform.position && current.name != current2.name)
                     {
                         toDestroy[arcub] = current2;
                         arcub = arcub + 1;
                         Debug.Log (current.name + " & " + current2.name);
                     }
 [...]

The error is

 NullReferenceException: Object reference not set to an instance of an object
 (wrapper stelemref) object:stelemref (object,intptr,object)

Referring to the line "toDestroy[arcub] = current2;" The objects acctually exist, because if I delete that line, the line "Debug.Log(current.name + " & " + "current2.name")" works as expected. I just can't understand why the compiler can't assign every GameObject in the array toDestroy.

I must use the variable "arcub" for the array, because I don't know how many objects the script will intercept... This is how I instantiated the variables on the top of the class of the script.

 public int arcub;
 public GameObject[] toDestroy;

I'm sure the error is caused by some stupid mistake I did, but I really can't figure it out... .-. Can someone help me please?