Can't Upload OPML

When I have been attempting to upload an OPML file I get prompted the following message:

Uncaught TypeError: Cannot read property ‘body’ of null

Stack trace:
TypeError: Cannot read property ‘body’ of null
at Object.onImportComplete (https 192.xxx.x.xxx/tt-rss/js/PrefHelpers.js?1587193593:211:33)
at HTMLIFrameElement.onload (https 192.xxx.x.xxx/tt-rss/prefs.php:1:14)

I don’t know exactly what is the cause of the problem or where to proceed from here.
I have tried to edit the OPML files by hand with no success.

  1. don’t post in development with non-development issues
  2. read support category sticky and provide all information mentioned there