Crash on ObjectMapper processDictionaryFromObject

I got some crash logs from Fabric on this method.

This is where the crash originates, basically converting an object into dictionary : https://github.com/ooni/probe-ios/blob/master/ooniprobe/Model/Settings/Settings.m#L19

Crash log : https://github.com/ooni/probe-ios/issues/223
Wasn't able to reproduce it on my own