NSInvalidArgumentException', reason: '-[__NSCFDictionary setObject:forKey:]: attempt to insert nil value (key: WebKitLocalStorageDatabasePathPreferenc

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];

NSDictionary *userDefaultsDictionary = [userDefaults dictionaryRepresentation];
NSString *strWebDatabaseDirectory = [userDefaultsDictionary objectForKey:@"WebDatabaseDirectory"];
NSString *strWebKitLocalStorageDatabasePathPreferenceKey = [userDefaultsDictionary objectForKey:@"WebKitLocalStorageDatabasePathPreferenceKey"];

[userDefaults removePersistentDomainForName:[[NSBundle mainBundle] bundleIdentifier]];

if (strWebDatabaseDirectory) {
[userDefaults setObject:strWebDatabaseDirectory forKey:@"WebDatabaseDirectory"];}
if (strWebKitLocalStorageDatabasePathPreferenceKey) {
[userDefaults setObject:strWebKitLocalStorageDatabasePathPreferenceKey forKey:@"WebKitLocalStorageDatabasePathPreferenceKey"];}

[userDefaults synchronize];