QTMovieModernizer errors - objective-c

Question

I'm trying to write a piece of code that would "modernize" a video file. This is the code :
NSOpenPanel *openPanel = [[NSOpenPanel alloc] init];
[openPanel setCanChooseFiles:NO];
[openPanel setCanChooseDirectories:YES];
if([openPanel runModal] == NSCancelButton)
return;
NSMutableString *destination = [[[openPanel URL] absoluteString] mutableCopy];
[destination appendString:#"/"];
[destination appendString:[self fileName:[url absoluteString] withExtention:NO]];
[destination appendString:#".m4v"];
QTMovieModernizer *modernizer = [[QTMovieModernizer alloc] initWithSourceURL:url destinationURL:[NSURL URLWithString:destination]];
[modernizer modernizeWithCompletionHandler:^{NSLog(#"Modernization complete to file %# - error : %#",[modernizer destinationURL],[modernizer error]);}];
The problem is that I get an NSInvalidArgumentException. I've checked the source and destination url carefully and they seem to be ok. The error seems to happen at the initialization the QTMovieModernizer instance. What can be going wrong here?
Error details :
NSInvalidArgumentException
(
0 CoreFoundation 0x00007fff82cdc25c __exceptionPreprocess + 172
1 libobjc.A.dylib 0x00007fff84c8ae75 objc_exception_throw + 43
2 QTKit 0x00007fff84fc4303 -[QTMovieModernizer createUniqueLegacyFilePath:extension:inDirectory:] + 0
3 MediaPlayer 0x0000000100003bb9 -[AppDelegate modernize:] + 1097
4 MediaPlayer 0x000000010000536f -[AppDelegate openFile:] + 879
5 AppKit 0x00007fff8339d340 -[NSApplication sendAction:to:from:] + 327
6 AppKit 0x00007fff833b82a8 -[NSMenuItem _corePerformAction] + 394
7 AppKit 0x00007fff833b7fe4 -[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:] + 117
8 AppKit 0x00007fff8340748d -[NSMenu _internalPerformActionForItemAtIndex:] + 35
9 AppKit 0x00007fff83407309 -[NSCarbonMenuImpl _carbonCommandProcessEvent:handlerCallRef:] + 104
10 AppKit 0x00007fff833ae0d6 NSSLMMenuEventHandler + 716
11 HIToolbox 0x00007fff8b7d21d4 _ZL23DispatchEventToHandlersP14EventTargetRecP14OpaqueEventRefP14HandlerCallRec + 892
12 HIToolbox 0x00007fff8b7d1787 _ZL30SendEventToEventTargetInternalP14OpaqueEventRefP20OpaqueEventTargetRefP14HandlerCallRec + 385
13 HIToolbox 0x00007fff8b7e5880 SendEventToEventTarget + 40
14 HIToolbox 0x00007fff8b81b640 _ZL18SendHICommandEventjPK9HICommandjjhPKvP20OpaqueEventTargetRefS5_PP14OpaqueEventRef + 420
15 HIToolbox 0x00007fff8b84e238 SendMenuCommandWithContextAndModifiers + 59
16 HIToolbox 0x00007fff8b84e1e0 SendMenuItemSelectedEvent + 178
17 HIToolbox 0x00007fff8b84e0bf _ZL19FinishMenuSelectionP13SelectionDataP10MenuResultS2_ + 94
18 HIToolbox 0x00007fff8b856095 _ZL14MenuSelectCoreP8MenuData5PointdjPP13OpaqueMenuRefPt + 718
19 HIToolbox 0x00007fff8b855cc1 _HandleMenuSelection2 + 446
20 AppKit 0x00007fff8332073c _NSHandleCarbonMenuEvent + 284
21 AppKit 0x00007fff8317f6be _DPSNextEvent + 2170
22 AppKit 0x00007fff8317ea2b -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 122
23 AppKit 0x00007fff83172b2c -[NSApplication run] + 553
24 AppKit 0x00007fff8315d913 NSApplicationMain + 940
25 MediaPlayer 0x0000000100008362 main + 34
26 libdyld.dylib 0x00007fff830d95fd start + 1
27 ??? 0x0000000000000003 0x0 + 3
)
EDIT :
I found out that -fileName:withExtension: returns a string by replacing %20 by spaces. After handling this, the exception at the initsilizing is gone but instead, I have the following error when converting the file :
<<<< QTMovieModernizer >>>> FigFormatUtilsCreateFormatReader: Error -12848 from FigFormatReaderCreateForStream
<<<< QTMovieModernizer >>>> formatReaderForSourceURL: Error in FormatReaderForSourceURL: -12848