iOS: Warning: Attempt to present on while a presentation is in

- (void)imagePickerController:(UIImagePickerController *)picker 
didFinishPickingMediaWithInfo:(NSDictionary *)info {

     // TODO: make this all threaded?
     // crop the image to the bounds provided
     img = [info objectForKey:UIImagePickerControllerOriginalImage];
     NSLog(@"orig image size: %@", [[NSValue valueWithCGSize:img.size] description]);

     // save the image, only if it's a newly taken image:
     if ([picker sourceType] == UIImagePickerControllerSourceTypeCamera) {
         UIImageWriteToSavedPhotosAlbum(img, nil, nil, nil);
     }

     // self.image_View.image = img;
     // self.image_View.contentMode = UIViewContentModeScaleAspectFit;

    NSLog(@"Picker has returned");
    [self dismissViewControllerAnimated:YES
                             completion:^{
                                ModalViewController *sampleView = [[ModalViewController alloc] init];
                                [self presentModalViewController:sampleView animated:YES];
                             }];
}