The full error is:
Uncaught TypeError: Cannot read property 'appendChild' of null at Drupal.AjaxCommands.actionLinkFlash (flag-action_link_flash.js?v=8.6.13:31)
My configuration is that this flag is set for flagging, but not unflagging. It seems that what's happening is that the parent div (or whatever) is being removed (because there should not be an unflag link) before the "flagged" message is shown for a short interval.
Closed: works as designed
4.0
User interface
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It's likely this issue predates Contrib.social: some issue and comment data are missing.