The docs say the progressBlock is called on a background queue, so when doing UI…

…, we need to dispatch on the main queue
* @param progressBlock  A block called while image is downloading
* @note the progress block is executed on a background queue

@@ -43,11 +43,13 @@
                           placeholderImage:nil
                                    options:SDWebImageProgressiveDownload
                                   progress:^(NSInteger receivedSize, NSInteger expectedSize, NSURL *targetURL) {
+                                      dispatch_async(dispatch_get_main_queue(), ^{
                                           if (!activityIndicator) {
                                               [weakImageView addSubview:activityIndicator = [UIActivityIndicatorView.alloc initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray]];
                                               activityIndicator.center = weakImageView.center;
                                               [activityIndicator startAnimating];
                                           }
+                                      });
                                   }
                                  completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
                                      [activityIndicator removeFromSuperview];