'-[__NSCFString replaceOccurrencesOfString:withString:options:range:]: nil argument'

2016-09-21 15:27:09.838 HandShaker[10945:178953] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString replaceOccurrencesOfString:withString:options:range:]: nil argument'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fffa118b4f2 __exceptionPreprocess + 178
	1   libobjc.A.dylib                     0x00007fff9d0b273c objc_exception_throw + 48
	2   CoreFoundation                      0x00007fffa11f24bd +[NSException raise:format:] + 205
	3   CoreFoundation                      0x00007fffa10d8f5c -[__NSCFString replaceOccurrencesOfString:withString:options:range:] + 156
	4   Foundation                          0x00007fff9790ad35 -[NSString stringByReplacingOccurrencesOfString:withString:options:range:] + 184
	5   Foundation                          0x00007fff9790ac6e -[NSString stringByReplacingOccurrencesOfString:withString:] + 92
	6   HandShaker                          0x0000000100037f40 -[SFViewModel setSffile:] + 2080
	7   HandShaker                          0x0000000100037370 -[SFViewModel initWithSFFile:withClient:] + 768
	8   HandShaker                          0x000000010017d3e1 -[SFPhotoAlbumsViewModel parserPhotoLibraryData:] + 1297
	9   HandShaker                          0x0000000100071eec __53-[SFPhotoViewController _reloadLibraryDataFromDevice]_block_invoke + 1084
	10  SmartFinderCore                     0x00000001004ad913 __44-[SSPManager fetchPhotoLibraryWithCallback:]_block_invoke + 1171
	11  SmartFinderCore                     0x0000000100538be0 __69-[SSPGetPhotoLibraryRequestOperation initWithSSPManager:andCallback:]_block_invoke.81 + 48
	12  libdispatch.dylib                   0x0000000100938070 _dispatch_call_block_and_release + 12
	13  libdispatch.dylib                   0x000000010092acc5 _dispatch_client_callout + 8
	14  libdispatch.dylib                   0x00000001009426ae _dispatch_main_queue_callback_4CF + 2845
	15  CoreFoundation                      0x00007fffa11409e9 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
	16  CoreFoundation                      0x00007fffa10ff8dd __CFRunLoopRun + 1949
	17  CoreFoundation                      0x00007fffa10feed8 CFRunLoopRunSpecific + 296
	18  HIToolbox                           0x00007fff92988935 RunCurrentEventLoopInMode + 235
	19  HIToolbox                           0x00007fff9298876f ReceiveNextEventCommon + 432
	20  HIToolbox                           0x00007fff929885af _BlockUntilNextEventMatchingListInModeWithFilter + 71
	21  AppKit                              0x00007fff9b646df6 _DPSNextEvent + 1067
	22  AppKit                              0x00007fff9b646226 -[NSApplication _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 454
	23  AppKit                              0x00007fff9b63ad80 -[NSApplication run] + 682
	24  AppKit                              0x00007fff9b604368 NSApplicationMain + 1176
	25  HandShaker                          0x0000000100017712 main + 34
	26  libdyld.dylib                       0x00007fff9e4a45ad start + 1
	27  ???                                 0x0000000000000003 0x0 + 3
)

找到以下代码，root为空，便会提示这个错误。

rootRelativePath = [folder stringByReplacingOccurrencesOfString:root withString:@""];