App terminate due to - uncaught exception 'NSGenericException', - iphone

Question

I got the below error while fetching some data from sever. While fetching data, i m displaying an custom alert view with activity indicator. Alert is using main thread while data fetching is done by NSThread. This problem occurs only at some time, not always. I m not able to understand what to do.
* Terminating app due to uncaught exception 'NSGenericException', reason: '* Collection was mutated while being enumerated.(
"",
"",
""
)'
* Call stack at first throw:
(
0 CoreFoundation 0x00f2abe9 __exceptionPreprocess + 185
1 libobjc.A.dylib 0x0107f5c2 objc_exception_throw + 47
2 CoreFoundation 0x00f2a6a9 __NSFastEnumerationMutationHandler + 377
3 UIKit 0x00371b8e -[UIView(Hierarchy) _findFirstSubviewWantingToBecomeFirstResponder] + 189
4 UIKit 0x00366a13 -[UIView(Hierarchy) _promoteDescendantToFirstResponderIfNecessary] + 42
5 MyApplnName 0x00015154 -[SecondViewController getDataFromServer] + 1798
6 Foundation 0x000a1d4c -[NSThread main] + 81
7 Foundation 0x000a1cd8 __NSThread__main__ + 1387
8 libSystem.B.dylib 0x928db7fd _pthread_start + 345
9 libSystem.B.dylib 0x928db682 thread_start + 34
)
terminate called after throwing an instance of 'NSException'

Answer 1

The problem is in part of your code where you enumerate some object, for example, NSMutableArray or NSMutableSet, using for-loop (for (id _obj in object)) and in that loop removing or adding some objects to the same object. It is not possible and causes exception.
You can replace that for-loop with for-loop using index variable:
for (int i=0; i<[object count]; i++)
{
//add or remove objects if you want
}