Last modified: 2014-06-02 23:28:45 UTC
This appears in a fresh installation of the Collection extension in a MediaWiki 1.19 installation. I've added in require_once( "$IP/includes/HTMLForm.php" ); so that the HTMLForm class could get instantiated, but alas, it seems that it doesn't. Is this an instance of "you need to upgrade the core MediaWiki"?
Resolved by upgrading mediawiki 1.22.7