Occurs on action=submit in onunload event.
e.g. https://fr.wikipedia.org/w/index.php?title=Handball_Club_de_Nantes&action=submit
Stack trace:
at ext.wikiEditor.js/</</window.onunload https://fr.wikipedia.org/w/load.php?lang=fr&modules=ext.wikiEditor&skin=vector&version=iemho:4:787