[iOS 13]: Fatal Exception: NSInvalidArgumentException Can't add self as subview ([NSCache init])
The app built with Xcode 11.3 & iOS SDK 13.3 doesn't have this crash anymore. I guess it was solved on the iOS SDK side.
UPDATE: 18.06.2020
For those, who continue having the issue, I'd recommend updating to the new Firebase Crashlytics from the old Fabric Crashlytics. The guide is here: https://firebase.google.com/docs/crashlytics/upgrade-sdk?platform=ios
Moreover, I got an email from Firebase about the old SDK deprecation:
We are writing to let you know that the Fabric Crashlytics SDK is now deprecated and will continue reporting your app's crashes until November 15, 2020. After this date, the Fabric Crashlytics SDK and beta versions of the Firebase Crashlytics SDK will stop sending crash reports for your app.