Help with arrays, multidimensional

Hey guys,

I'm having a real issue grasping the concept of this so I hope I explain it well. I have searched though other answers on here and tried to implement an array in a container class instead of using a traditional 2D array for my tile manager.

I'm pretty much tearing my hair out trying to get it working, this is what I have;

 [System.Serializable]
 public class Rows
 {
     public int[] cols;
 };
 
 public class ArrayTest : MonoBehaviour {
     
     public Rows[] rows;
     
     void Start()
     {
         rows = new Rows[3];
         
         rows[1].cols[1] = 100;
     }
 
 }

I've tried using a for loop to fill the elements, I've tried foreach. Everytime I get a NullReferenceException because while it initialised my 3 rows it never add anything to the cols.

Any help would be greatly appreciated.

EDIT: For clarification I want this to work in edit mode so I can size up, click and button and it'll build me a tile grid. I have this working with a 2D array but I can't seem to initialise the cols aspect of this in edit mode.