Hi, Bruce. I think that your best bet would be to install a breadboard edge. Poplar is usually pretty stable but stresses can be introduced during the growing that can cause you grief. Since you know that this wood is succeptible, and you haven't finished, a breadboard edge can fix the problem and keep it from happening in other places. You will want to make the dados that form the tongues with a router so you follow any warp that remains after you cut the top back and use the edge to remove the warp. Don't forget to elongate the dowel holes in the tongue to allow the top to expand and contract across the tops width. If you do not allow the top to move across it's width, it will crack eventually.