error while trying to upload image with NSURLSession - ios

Question

I'm using the following code to upload and image:
- (IBAction)uploadPic:(UIButton *)sender {
PHFetchResult *result = [PHAsset fetchAssetsWithALAssetURLs:#[self.imageReferenceURL] options:nil];
PHAsset *asset = [result firstObject];
if (asset) {
PHImageManager *manager = [PHImageManager defaultManager];
[manager requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
// upload the `imageData`
NSURL *fileURL = info[#"PHImageFileURLKey"];
NSString *filename = [fileURL lastPathComponent];
NSString *mimeType = [self mimeTypeForPath:filename];
NSString *urlString = #"url";
NSMutableURLRequest* request= [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]];
[request setHTTPMethod:#"POST"];
NSString *boundary = #"---------------------------14737809831466499882746641449";
NSString *contentType = [NSString stringWithFormat:#"multipart/form-data; boundary=%#",boundary];
[request addValue:contentType forHTTPHeaderField: #"Content-Type"];
NSMutableData *postbody = [NSMutableData data];
[postbody appendData:[[NSString stringWithFormat:#"\r\n--%#\r\n",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
[postbody appendData:[[NSString stringWithFormat:#"Content-Disposition: form-data; name=\"uploaded_file\"; filename=\"%#\"\r\n", filename] dataUsingEncoding:NSUTF8StringEncoding]];
[postbody appendData:[[NSString stringWithFormat:#"Content-Type: %#\r\n\r\n", mimeType] dataUsingEncoding:NSUTF8StringEncoding]];
[postbody appendData:imageData];
[postbody appendData:[[NSString stringWithFormat:#"\r\n--%#--\r\n",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
NSURLSessionTask *task = [[NSURLSession sharedSession] uploadTaskWithRequest:request fromData:postbody completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(#"Response %#",responseString);
}];
[task resume];
}];
}
}
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
UIImage *chosenImage = info[UIImagePickerControllerEditedImage];
self.imageView.image = chosenImage;
self.imageReferenceURL = info[UIImagePickerControllerReferenceURL];
[picker dismissViewControllerAnimated:YES completion:nil];
}
and I got this error:
2015-09-30 10:35:42.921 udazz[874:453021] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]'
*** First throw call stack:
(0x183804f5c 0x198307f80 0x1836ee30c 0x1836f9db4 0x100020240 0x188d923e4 0x188d92360 0x188d7ac88 0x188d91c78 0x188d918a8 0x188d8aadc 0x188d5ba2c 0x188d59f18 0x1837bc5a4 0x1837bc038 0x1837b9d38 0x1836e8dc0 0x18e83c088 0x188dc2f60 0x1000315a4 0x198b328b8)
libc++abi.dylib: terminating with uncaught exception of type NSException