Safe can be a pretty relative term. Many of our foods are "safe" and can still cause long-term health issues if we continue eating them. I'd stick with their natural diet and things that they're likely to encounter by chance either directly or indirectly. I'd avoid any food products that have been processed as most of those contain preservatives or "safe" additives. Fruit would be the best way to go as mantids are most likely to contact that sort of food item through incidental contact when catching frugivorous prey. As for non-insect protein sources, I'd probably only stick with pollen. Proteins can be quite complex and some cannot be digested if the organism ingesting it lacks the proper enzymes to process it. Lunch meats are probably something you'd want to avoid not only because they're a protein source foreign to mantids, but because of the additives used to preserve it.
Small mice, young snakes, lizards, hummingbirds and fish can also make part of the menu. Well, only the biggest mantis species can consume some on that list.