If you're taking the Eurostar (not sure about Thalys), after you clear security & passport control but prior to boarding, you'll find yourself in another waiting area. On the left near the boarding point, there's a small shopping area and food court that's only available to those that have cleared security. This should be open until just prior to departure.

Having been through Midi a few times over the past month, I haven't experienced anything that would make me want to avoid the place, nor do I fear theft there more than any other train station. Like always when traveling, be smart, be vigilant, and be aware of your surroundings and you'll be fine.