My opinion would be to seal it first. Some pieces of poplar take stain really well while others pieces do not. If you look at the pictures of the pie boxes, you can see where some of the poplar did not stain evenly while other pieces did.
https://forum.shopsmith.com/viewtopic.php?p=275518#p275518 I would also suggest your wife try out her chosen stain on a couple of pieces of poplar to see how it looks both with and without sealer.