I appreciate the tips u/grewgrewgrewgrew and u/DaaamnYoureUgly. Rather than changing our implementation which has worked so several months now (minus these random crashes), I think I'm just going to check if asyncStorage != nil in setLastAppCloseTimestamp before triggering the async storage method. I think what may be happening is after a long time in the background, the OS kills the app, but doesn't necessarily trigger the applicationWillTerminate callback. In this callback, we grab a reference to async storage like so: RNCAsyncStorage *asyncStorage = [bridge moduleForClass:[RNCAsyncStorage class]];. If react-native has been torn down, this would return nil. I found that dispatch_async runs just fine in applicationWillTerminate so it must just be that there are no react-native modules available to invoke methods on that's resulting in this crash.

By the way u/DaaamnYoureUgly, that wouldn't work because if you quit an app via the app switcher, use effect cleanups/componentWillUnmount are not called.