fix crash by adding class check on captionView
fix crash by adding class check on captionView

https://rink.hockeyapp.net/manage/apps/152731/app_versions/719/crash_reasons/244200410

josve05a/wikipedia-ios,josve05a/wikipedia-ios,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,wikimedia/wikipedia-ios,wikimedia/wikipedia-ios,montehurd/apps-ios-wikipedia,wikimedia/wikipedia-ios,wikimedia/apps-ios-wikipedia,josve05a/wikipedia-ios,josve05a/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/wikipedia-ios,montehurd/apps-ios-wikipedia,wikimedia/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,wikimedia/wikipedia-ios,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,josve05a/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/wikipedia-ios