Object not set to an instance of an object, but the variable is already declared

hi, look at that :

 var listOfFloors : Array[ , ];
 
 var firstFloor : GameObject;
 
 
 var objs : GameObject[];
 
 private var hit : RaycastHit;
 
 var isOk1 = false;
 var isOk2 = false;
 
 var floorsToList : GameObject[];
 
 function Start()
 {
 
 
 floorsToList = gameObject.FindGameObjectsWithTag("floor");
 var tx : float;
 var ty : float;
 var fx : float;
 var fy : float;
 
 fx = firstFloor.transform.position.x;
 fy = firstFloor.transform.position.z;
 
 for(m = 0; m < 15; m++)
 {
     for(i = 0; i < 10; i++)
     {
         tx = fx + 5*m;
         ty = fy + 5*i;
         
         
         var positionT = new Vector3(tx,0,ty);
         objs = GameObject.FindSceneObjectsOfType(typeof(GameObject));
         
         
         for (go in objs)
         {
           if (go.transform.position == positionT)
           {
             listOfFloors[m, i] = new Array(go, m, i);
             Debug.Log("a cube has been saved");
             break;
           }
         }
     }
 }
 
 lastFloor = listOfFloors[startEnemy[0], startEnemy[1]];
  }

I don't understand, this generate an error "Object not set to an instance of an object" (at line 54 & 122) These objects are in the scene.

Thanks