This happens when navigating back and forth around https://ka.wikiquote.org/w/index.php?search=sf&title=Special:Search&go=Go. It's hard to reproduce by direct url but going to the search bar, entering some text, submitting the search and going back consistently triggers.
Exception in load-callback in module mediawiki.widgets: Error: Unknown widget type: id: searchText, class: mw.widgets.SearchInputWidget
Might be caused by a local modification of sorts.