How to figure out why my app is crashing?

问题

I have an iOS app that is on the app store at the moment. It works fine, just as expected, on 99% of the Apple devices.

However, there are some devices where it opens once, flashes, and then closes again after first use.

I don't think it is a device specific issue as this was experienced on iPhones 5 and 6S plus and at the same time it works perfectly fine on other iPhone 5 and 6Ss. Similarly, same thing on the software as well (it is upto date with 11 and crashes on 11 and crashed on previous versions as well).

How can I figure out why this is happening?

This is the error :

If I was to replace it with this code, how would I know if it works :

 switch tempDayHolder
                    {
                    case "Monday":
                        outputWeekdays.append(2)
                    case "Tuesday":
                        outputWeekdays.append(3)
                    case "Wednesday":
                        outputWeekdays.append(4)
                    case "Thursday":
                        outputWeekdays.append(5)
                    case "Friday":
                        outputWeekdays.append(6)
                    case "Saturday":
                        outputWeekdays.append(7)
                    case "Sunday":
                        outputWeekdays.append(1)
                    default :
                        outputWeekdays.append(1)
                    }

来源：https://stackoverflow.com/questions/47036594/how-to-figure-out-why-my-app-is-crashing