NullReferenceException: Object reference not set to an instance of an object (int[,])

Hello, I am making simple game where I need to use two dimensional array, so I am using int[,].

Then on the scene load I am assigning values like this:

 //GameBoardController.js
 function getMatrix(){
   for (var i = 0; i < numrows; ++i){
     var columns: Array = [];
       for (var j = 0; j < numcols; ++j){
         activeCells[i,j] = 0;
       }
     }
   }
 }

When I print print(activeCells) now, i get System.Int32[,] which is expected result.

But then in another function which is triggered by button:

 //GameBoardController.js
 function resetScene(){
   for(var x = 0 ; x < sizeX ; x++){ //sizeX is as public int variable
     for(var y = 0 ; y < sizeY ; y++){ //sizeY is as public int variable
       activeCells[x,y] = 0;
     }
   }
 }

And here is the problem. Function resetScene() is failing and giving me error message:

 NullReferenceException: Object reference not set to an instance of an object
 GameBoardController.resetScene () (at Assets/Scripts/GameBoardController.js:834)

But this is wrong because in the same script few lines above resetScene() I am assigning and then printing activeCells and everything is working.

Does please anyone know what am I doing wrong? Should I attach another GameBoardController script to the button? GameBoardController is attached to empty GameObject and everything but this works.