How to use Instruments to determine part of the code that crashes-Collection of common programming errors
As per your error log, in my understanding you trying to insert nil value in NSDictionary/NSMutableDictionary. You can not pass nil as value or key into dictionary otherwise your code will crash. My suggestion you debug you code using breakpoint, trace you execution stack i believe you will get your exact solution where your nil value inserting in dictionary.
If you want to find exact line of crash you can take help of debugger command. Enter bt (back trace command) in debugger window, it will give also same executed crash stack that give you instrument analyzer. Go through following links
https://developer.apple.com/library/ios/documentation/DeveloperTools/Conceptual/WhatsNewXcode/Articles/xcode_5_0.html