"Cancel" button did not work to cancel processing

I finally was able to get the interface to work and transferred 272 images for a map from Google Drive. Somehow, one of the images failed to upload. When the rest of the images had finished uploading, a dialog box came up giving me the option to either "cancel" or "continue with processing anyway" ( or something to that effect). Not wanting a hole in my map, I clicked on the red "Cancel" button on the left. 

But it didn't cancel. I got forwarded to the page that shows the map progress and will now have a map with a hole in it.

Why did that not cancel when "cancelled"?