Why am I getting a `unrecognized selector sent to instance` error?

Question

I have a Ruby on Rails application that has an API, it's an OAuth 2.0 provider and uses Doorkeeper. I am creating an iPhone client for that application and am using the gtm-oauth2 library for authentication. Here is the Github repository for the iPhone app.

I manage do the authentication request using the library and get the response from the OAuth server, but (I think that) when the iPhone app receives the response, the iPhone app crashes. I get the following error:

2013-03-25 07:30:51.563 Catapult for iOS[68917:c07] -[NSNull length]: unrecognized selector sent to instance 0x14f2678
2013-03-25 07:30:51.564 Catapult for iOS[68917:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance 0x14f2678'
*** First throw call stack:
(0x13c1012 0x11e6e7e 0x144c4bd 0x13b0bbc 0x13b094e 0xf074 0x185e3 0x13b51bd 0x13b50d6 0x1531a 0x1512b 0x14ae2 0x13b51bd 0x13b50d6 0x11d0a 0x1032a 0x13b51bd 0x13b50d6 0x79be 0x77ed 0x8cf2 0xcec589 0xcea652 0xceb89a 0xcea60d 0xcea785 0xc37a68 0x4a2a911 0x4a29bb3 0x4a67cda 0x13638fd 0x4a6835c 0x4a682d5 0x4952250 0x1344f3f 0x1344a39 0x1367734 0x1366f44 0x1366e1b 0x22be7e3 0x22be668 0x12affc 0x2c6d 0x2b95)
libc++abi.dylib: terminate called throwing an exception

I am a complete noob/beginner when it comes to iOS development and Objective-C programming, and I am learning how to create my first app using this little project. I asked the same question in the gtm-oaut2 Google Group and according to them, the problem comes from my code and not the library. The problem is that all of my code is taken from their wiki and I can't pinpoint where the app crashes. If I understand correctly, at some point I am calling length of NSNull, but I am not calling length anywhere, hence my confusion. Also, when the app crashes, the line hilighted is in the main.m file and the error (on the right label, not in the output) is the following:

Thread 1: signal SIGABRT

I have no clue what that means...

Does anyone know what the problem might be please?

Answer 1

You're right that the issue is that somewhere you've got the length message being sent to an instance of NSNull . To pinpoint where exactly this is happening set a breakpoint on objc_exception_throw. You can do this in the Xcode UI with little "+" button in the bottom left corner on the breakpoints tab. Select "Add Exception breakpoint." Then the debugger will stop your program at the point where the error occurs instead of waiting for the program to actually crash.

Answer 2

Well... you understood the error correctly.. You are somewhere calling length on a NSNull instance.

I must find out where this is happening. I think this is happening in a JSON response. Maybe you (or "the code") is calling length on something it expects its a NSString but it is instead NSNull (that is the json has a NULL value). Try if you manage to print the json response and see if you found a key mapped to a null value...

Answer 3

NSNull does not respond to length

You can check the documentation for NSNull to see that this is the case.

Without having an idea of what your code base is doing I am not sure where to look, you must be calling [NSNull null]; at some point to get the NSNull object or you are using a framework somewhere that returns this.