Yesterday, every third-party app I tried to launch crashed as it started up. Every one! My Apple apps (Podcasts, Mail, App Store, etc) all started without a hitch.
The short answer: I rebooted my iPhone, turning it all the way off and back on again, and the problem went away. I also noticed that my iPhone took longer to boot up than usual (or I was impatient.)
Before I realized that every app was crashing on launch, I fixed the issue for the first app by deleting and reinstalling the app. I was lucky that the app operates from cloud data, so deleting the app wasn't a big deal.
I don't recommend deleting and reinstalling because deleting an app deletes all of your stored data! So, try rebooting first, and if that doesn't work or the problem is with only one app, deleting and reinstalling may help. But again, note that deleting an app will delete all of it's data from your iPhone. This is not an issue if the app signs into a cloud account to get your data.