Xcode exception interpretation - objective-c

Question

Can anyone tell me how to tackle this exception? The debugger is at an exception breakpoint and as you can see there seems to be zero actionable information about the exception. Where did it occur? Pretty sure it's not in my code but I'm at a loss about how to proceed...
Many thanks for any help.
Edit 1:
Here's the console contents
* Terminating app due to uncaught exception 'NSRangeException', reason: '*
-[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array'
*** First throw call stack: (0x30ce188f 0x3627e259 0x30c2a9db 0x8d029 0x915c5 0x35723c59 0x35725ee7 0x30cb42ad 0x30c374a5 0x30c3736d 0x36ffb439 0x32917cd5 0x72e4d 0x72de8)
terminate called throwing an exception
Edit 2:
Here's the backtrace
thread #1: tid = 0x1c03, 0x3627e238 libobjc.A.dylib`objc_exception_throw, stop reason = breakpoint 1.1
frame #0: 0x3627e238 libobjc.A.dylib`objc_exception_throw
frame #1: 0x30c2a9da CoreFoundation`-[__NSArrayM objectAtIndex:] + 270
Edit 3 (the solution):
Here's the backtrace using the main() approach
2012-08-31 10:27:21.489 <>[820:707] Uncaught exception *** -[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array
2012-08-31 10:27:32.908 <>[820:707] Stack trace: (
0 CoreFoundation 0x30ce18a7 __exceptionPreprocess + 186
1 libobjc.A.dylib 0x3627e259 objc_exception_throw + 32
2 CoreFoundation 0x30c2a9db -[__NSArrayM objectAtIndex:] + 270
3 <> 0x00050ed1 __55-[SFSummaryCard retrieveAndDisplayFirstImageForString:]_block_invoke_0 + 148
4 <> 0x0005546d __block_global_1 + 40
5 libdispatch.dylib 0x35723c59 _dispatch_call_block_and_release + 12
6 libdispatch.dylib 0x35725ee7 _dispatch_main_queue_callback_4CF$VARIANT$mp + 194
7 CoreFoundation 0x30cb42ad __CFRunLoopRun + 1268
8 CoreFoundation 0x30c374a5 CFRunLoopRunSpecific + 300
9 CoreFoundation 0x30c3736d CFRunLoopRunInMode + 104
10 GraphicsServices 0x36ffb439 GSEventRunModal + 136
11 UIKit 0x32917cd5 UIApplicationMain + 1080
12 <> 0x00036c01 main + 220
13 <> 0x00036b20 start + 40
)
The key element being: retrieveAndDisplayFirstImageForString()

Answer 1

There is a problem on Xcode 4/iOS 5 that sometimes you don't get the exception traceback in the emulator, due to an apparent bug in the emulator's pseudo OS. The solution is to add an explicit traceback in your main, along the lines of
#try {
retVal = UIApplicationMain...
}
#catch (NSException* exception) {
NSLog(#"Uncaught exception %#", exception);
NSLog(#"Stack trace: %#", [exception callStackSymbols]);
}

Answer 2

You can make exception breakpoint print backtrace in console.
Example:
-(void)backtraceTest
{
NSArray *array = [NSArray arrayWithObjects:#"1", #"2", nil];
id obj = [array objectAtIndex:3]; // out of bounds
NSLog(#"%#", obj);
}
gives this backtrace:
You can see that at line 34 of HelloWorldLayer.m file objectAtIndex method of NSArray was called and then exception happened.

Answer 3

This is relevant:
http://ijoshsmith.com/2011/11/28/debugging-exceptions-in-xcode-4-2/
You can add an "exception breakpoint" that shows backtraces; event handier, you can move it to a "user breakpoint" that appears in all your projects.
Wish I had learned this trick earlier.

Answer 4

I was able to resolve by workaround:
Closed all open applications
relaunch Finder
run sudo find / -name ".DS_Store" -exec rm {} \;
this removes all .DS_Store old files on the computer
launch finder and set default view to "LIST VIEW" as shown here: http://macs.about.com/od/usingyourmac/ss/Setting-Finder-Views-For-Folders-And-Su b-Folders_2.htm
rerun the offending application and file chooser works