App is crashing in while calculating

[Jagadeeshwar-Reddy]

2013-12-18 18:29:42.391 MGWordCounterDemoMobile[14958:3603] NSScanner: nil string argument
2013-12-18 18:29:42.395 MGWordCounterDemoMobile[14958:3603] NSScanner: nil string argument
2013-12-18 18:29:44.187 MGWordCounterDemoMobile[14958:1303] NSScanner: nil string argument
2013-12-18 18:29:44.492 MGWordCounterDemoMobile[14958:1303] NSScanner: nil string argument
2013-12-18 18:31:18.921 MGWordCounterDemoMobile[14958:4a13] NSScanner: nil string argument
2013-12-18 18:31:19.026 MGWordCounterDemoMobile[14958:4a13] * Terminating app due to uncaught exception 'NSRangeException', reason: '* -[NSConcreteScanner setScanLocation:]: Index 18446744073709551599 out of bounds; string length 0'
*** First throw call stack:
(
0 CoreFoundation 0x000000010256d795 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x00000001015f5991 objc_exception_throw + 43
2 CoreFoundation 0x000000010256d5ad +[NSException raise:format:] + 205
3 Foundation 0x0000000100fb344c -[NSConcreteScanner setScanLocation:] + 118
4 MGWordCounterDemoMobile 0x0000000100004f45 -[MGWordCountOperation main] + 1605
5 Foundation 0x00000001010349e4 -[__NSOperationInternal _start:] + 623
6 Foundation 0x00000001010368bb __NSOQSchedule_f + 64
7 libdispatch.dylib 0x0000000105bdc6fd _dispatch_client_callout + 8
8 libdispatch.dylib 0x0000000105bcaf2f _dispatch_async_redirect_invoke + 174
9 libdispatch.dylib 0x0000000105bdc6fd _dispatch_client_callout + 8
10 libdispatch.dylib 0x0000000105bccb83 _dispatch_root_queue_drain + 362
11 libdispatch.dylib 0x0000000105bccd6b _dispatch_worker_thread2 + 40
12 libsystem_pthread.dylib 0x0000000105f86ef8 _pthread_wqthread + 314
13 libsystem_pthread.dylib 0x0000000105f89fb9 start_wqthread + 13
)
libc++abi.dylib: terminating with uncaught exception of type NSException