ios - How to use URLDecode?

Question

I'm trying to get access token,I'm following this link http://www.stevesaxon.me/posts/2011/window-external-notify-in-ios-uiwebview/ to get that,but I'm getting some problem when decode URL.Please go through that link.

- (NSDictionary*)parsePairs:(NSString*)urlStr
{
    NSRange r = [urlStr rangeOfString:@"="];
    if(r.length == 0)
    {
        return nil;
    }

    //Here Program received signal stopped
    NSString* token = [[urlStr substringFromIndex:r.location + 1 ] URLDecode];

    NSCharacterSet* objectMarkers;
    objectMarkers = [NSCharacterSet characterSetWithCharactersInString:@"{}"];
    token = [token stringByTrimmingCharactersInSet:objectMarkers];

    NSError* regexError;
    NSMutableDictionary* pairs = [NSMutableDictionary dictionaryWithCapacity:10];

    NSRegularExpression* regex;
    regex = [NSRegularExpression regularExpressionWithPattern:@"\"([^\"]*)\":\"([^\"]*)\""
                                                      options:0
                                                        error:&regexError];
    NSArray* matches = [regex matchesInString:token
                                      options:0
                                        range:NSMakeRange(0, token.length)];

    for(NSTextCheckingResult* result in matches)
    {
        for(int n = 1; n < [result numberOfRanges]; n += 2)
        {
            NSRange r = [result rangeAtIndex:n];
            if(r.length > 0)
            {
                NSString* name = [token substringWithRange:r];

                r = [result rangeAtIndex:n + 1];
                if(r.length > 0)
                {
                    NSString* value = [token substringWithRange:r];

                    [pairs setObject:value forKey:name];
                }
            }
        }
    }

    regex = [NSRegularExpression regularExpressionWithPattern:@"\"([^\"]*)\":([0-9]*)"
                                                      options:0
                                                        error:&regexError];
    matches = [regex matchesInString:token
                             options:0
                               range:NSMakeRange(0, token.length)];

    for(NSTextCheckingResult* result in matches)
    {
        for(int n = 1; n < [result numberOfRanges]; n += 2)
        {
            NSRange r = [result rangeAtIndex:n];
            if(r.length > 0)
            {
                NSString* name = [token substringWithRange:r];

                r = [result rangeAtIndex:n + 1];
                if(r.length > 0)
                {
                    NSString* value = [token substringWithRange:r];
                    NSNumber* number = [NSNumber numberWithInt:[value intValue]];

                    [pairs setObject:number forKey:name];
                }
            }
        }
    }

    return pairs;
}

Any ideas? Thanks in advance.