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There are a lot of questions on Stack Overflow related to this error. I’ve read some excellent posts on how Optionals work and this error in particular. However, I haven’t found any information about the best way to figure out which value is being set to nil and causing the error in the first place.
Are there any good debugging techniques to find out which Optional is causing this error?
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In order to access an optional's value (if it has one at all), you need to unwrap it. An optional value can be unwrapped safely or forcibly. If you force-unwrap an optional, and it didn't have a value, your program will crash with the above message. Xcode will show you the crash by highlighting a line of code.
It means that something was nil where it should not be. how I can fix the error. Check all the place you use Implicitly Unwrapped Optional type (!) and forced unwrapping (!). In your code shown, there are no forced unwrappings, so you may have some properties with Implicitly Unwrapped Optional type.
Force unwrapping an optional that does not contain a value throws a runtime error, crashing your application. The exclamation mark warned you this could happen, though.
Unwrap an optional type with the nil coalescing operator If a nil value is found when an optional value is unwrapped, an additional default value is supplied which will be used instead. You can also write default values in terms of objects.
Here's at least half an answer (would the other respondents please read the question first!): Use the simulator instead of an actual iOS device.
The debugger seems to be pretty good at pointing to the line with the maltreated optional that's causing the trouble... unless you are like me, choosing to run the code on an iOS device directly from time to time. In the latter case, the debugger lands me right in the middle of some assembler code with no sensible stack trace. Switching back to the simulator gives the exact line of code at fault.
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