How to best handle production errors?

I have a startup that I'm working on by myself and recently had what I would classify as a "SEV 1 Production Incident" from a tiny feature update that greets the user when you login.

In the mobile app if a user would login the screen would instantly go grey and no action could be performed again, even if the user exited and reopened the app.

The issue occurred due to an exception being raised for a string function that was called.

Claude 3.7 wrote that quick greeting function and I didn't pick up on any issues as in other languages (like JavaScript) there wouldn't be an exception for that specific string function (subString).

But in Dart (the app is made in Flutter) it raises an out of bounds error.

How I handled it
- Deployed a hot-fix via shorebird patching (Codepush for Flutter)
- Sent an email to all affected users with the fix (Shorebird requires users to exit and reopen the app to apply the fix)
- Updated the welcome email, website to alert people of the issue & a Threads post
- Sent an immediate App Store update with the true fix

My take aways
- Compartmentalise UI components so 1 breaking won’t break the entire app
- Add sentry to log errors so I get emails when this happens (current error logging just gets sent to the server but with no email or detailed info)
- Add a push notification service so users can be informed of any fixes/issues straight away
- E2E tests might be a good idea

Luckily my app was only recently released so not many users were affected and the true fix got approved quickly.

But, what are some ways that you have addressed high severity prod bugs and could I have done anything differently?

Thanks!