While devices as old as the iPhone 5s have been treated quite well by iOS 12, the same cannot be said for Fortnite. The smash hit has had a rocky run on the latest iOS betas, with a known crashing issue featured on each beta update's release notes. That changes with developer beta 4, as Apple confirms Fortnite is fixed.
Apple released iOS 12 developer beta 4 on Tuesday, July 17. In the release notes, under the "3rd Party Apps" section, "Fortnite might unexpectedly quit during use" is listed as a resolved issue, indicating the problems players have faced throughout the iOS 12 beta are, fortunately, no more.
Unfortunately, this patch is only contained to the developer beta at the moment. The public beta is still on version 2, which did not issue a fix for the Fortnite bug. Public beta testers will need to wait for iOS 12 public beta 3 before seeing these results. We expect Apple to seed public beta 3 in just a few days, one week at the latest.
So if this is the only reason you haven't jumped on the iOS 12 beta, you can now (or will be able to very soon).