If you are see "Missing frame that shows where the crash occurred" under Crash Details, please check the CRASH_REPORT_PROVIDER setting in your config; https://embrace.io/docs/ios/integration/crash-report/?_highlight=crashlytics#setting-up-the-crash-reporter. This message will appear if Firebase with Crashlytics and Embrace are enabled at the same time.
If you would like to use both crash reporters, The CRASH_REPORT_PROVIDER setting in your config should be set to Crashlytics. After updating the config to the correct setting, you should see a more helpful description of the crash.
This does not apply to Android.