Use stops you'll probably be fine. The problem is when you set stops and limits way below the current price ie during the main chunk of the crash. Then your order is triggered but you won't get filled due to immense order flow delays, broker and exchange side. I knew people on that day trying to get long on the bounce, just could not get filled. The only other thing you can do is to trade with a put option always on deep otm but really it's probably not worth it. Like insurance. Rules have changed since 2010 - not saying it can't happen again in HFT world but not as easily
Flash crashes rarely, if ever, occur when the market is in an uptrend. I have said this before, if you are worried about holding overnight, it usually means that you don't understand the market at a high level and/or you have too much exposure.