I don't think there is anything we can do.
The possibility that an application with external overlays applied would crash after update is documented in the Swift Themes documentation
When an app that has an overlay applied on it gets updated, there's very huge chances that it will start crashing. This is how dynamic overlays work and you are now aknowledging how these kind of crashes are not caused by bad coding in the themes. The best way you can keep things stable is by keeping automatic updates for all apps turned off on the Play Store. It will be your responsibility for apps crashing if you refuse to turn off automatic updates or deliberately update apps that have an overlay on top without then rebuilding the overlay accordingly, removing old overlays on the app, and ultimately remove all overlays on the app if the new update is causing issues, and patiently waiting for the new app version to be supported, if possible.
When you keep automatic updates turned off, we advise updating the theme first when you see it in the updates pending list. Check the changelog and check for the new supported versions of apps if there are any, and then you can update your other themeable apps if those versions are now supported by the update, if you want them themed of course.
Asked about this issue on Swift Themes official Telegram and got from an admin that
yes, the crash would happen because the overlay was made for Firefox v67, but users would update to v68 before it was supported by the theme/would update the browser and not update the overlay. this is fixed in the current iteration of the themes now as long as the overlay was properly updated
And indeed the crashes seem to disappear now based on users updating the overlays for Firefox 68.