App crashes when undoing a 'Replace All' function

[raspberrygable]

What happened?

iOS 26.1: The app (full paid version) crashes after attempting to undo a Find + Replace using Regex.
Have not yet reproduced such a crash resulting from any other triggers.

What are the steps to reproduce?

1. Successfully ran Find + Replace (Regex).
2. Hitting 'undo' afterwards crashed the app.
3. Reopened the app
4. File I'd just been working on was not visible in 'Recent Files' pane; had to manually locate it
5. When file located + reopened: changes had reverted to pre-Find-and-Replace state (i.e. the 'undo' function appears to have worked, but triggered the crash in some way).

What is the expected behavior?

I hope that Runestone can be updated to tolerate the user applying 'undo' following Find + Replace functions (or any other functions) reliably and without crashing.