If an app on your iPhone closes unexpectedly read this support article for what to do --> If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
From: Quit and reopen an app on iPhone - Apple Support (select the correct iOS version)
If an app isn't responding, you can quit it and then reopen it to try and resolve the issue. (Typically, there's
no reason to quit an app; quitting it doesn't save battery power, for example.)
1. To quit the app, open the App Switcher ( Open the App Switcher - Apple Support ), swipe right to find the app, then swipe up on the app.
2. To reopen the app, go to the Home Screen ( Go to the Home Screen - Apple Support ) (or App Library - https://support.apple.com/guide/iphone/find-your-apps-in-app-library-iph87abad19a), then tap the app.
If quitting and reopening the app doesn't resolve the issue, try restarting your iPhone. Refer to this article about how to turn an iPhone on or off —> Turn iPhone on or off - Apple Support