The Build Mode button works by initially hiding all active grids, and then toggle their visibility via the onclick event of the button. It makes perfect sense to initially hide all of your GridSquare objects in the scene to begin with, as if they are active to begin with, you will see them for a split second before this script hides them on start.
Assign your build menu parent here to hide all of its children onclick.
Hide grid objects with cells
If enabled, this will not only hide the grid on click, but also hide all the objects placed on the grids as well.