It seems now that if you delete all the slabs the hearthlings are no longer able to build the window. You will need to leave one slab somewhere in order to re-select the building and get a working build button.
So, find a spot where you don’t mind having a slab, a hole in the wall or in the floor for example. Connect it to the window hole in the usual fashion. Place the window and delete the connection to that slab. Select the slab to get the build button back and click “build”.
Relyss’ mod slabs with terrain colours may be of help with those offending last slabs.
Slabs with terrain colours
I hope this helps, Kyth.