Why is the value of the generated class null?

Question

Unity

private void Start()
    {
        grid = new Grid(100, 100, 10);
    }

if (Input.GetMouseButtonDown(0))
            {
                int width = grid.GetWidth(); <-- grid = null
            }

PS In general, I scored on initialization through a script and just made a GameObject with a script and without a constructor, now everything is fine.