From the apple documentation "An NSURL object initialized with URLString. If the URL string was malformed or nil, returns nil." Your stringURL isn't a correct formed url. For reference: https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html#//apple_ref/occ/clm/NSURL/URLWithString:
What you actually want to use is: fileURLWithPath: isDirectory:
instead.
NSURL *url = [NSURL fileURLWithString:[stringURL stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding] isDirectory:YES];
NSLog(@"%@", url);