Tried to enable "page previews" in Special:Preferences on my local test wiki, just freshly updated. On save, got this exception:
```
[63433c23a78794244a339fe8] /wiki/Special:Preferences TypeError from line 80 of /vagrant/mediawiki/extensions/Popups/includes/UserPreferencesChangeHandler.php: Argument 2 passed to Popups\UserPreferencesChangeHandler::onPreferencesFormPreSave() must be an instance of PreferencesForm, instance of PreferencesFormLegacy given, called in /vagrant/mediawiki/includes/Hooks.php on line 177
Backtrace:
#0 /vagrant/mediawiki/includes/Hooks.php(177): Popups\UserPreferencesChangeHandler::onPreferencesFormPreSave(array, PreferencesFormLegacy, User, boolean, array)
#1 /vagrant/mediawiki/includes/Hooks.php(205): Hooks::callHook(string, array, array, NULL)
#2 /vagrant/mediawiki/includes/preferences/DefaultPreferencesFactory.php(1693): Hooks::run(string, array)
#3 /vagrant/mediawiki/includes/preferences/DefaultPreferencesFactory.php(1724): MediaWiki\Preferences\DefaultPreferencesFactory->saveFormData(array, PreferencesFormLegacy)
#4 /vagrant/mediawiki/includes/preferences/DefaultPreferencesFactory.php(1525): MediaWiki\Preferences\DefaultPreferencesFactory->submitForm(array, PreferencesFormLegacy)
#5 /vagrant/mediawiki/includes/htmlform/HTMLForm.php(664): MediaWiki\Preferences\DefaultPreferencesFactory->MediaWiki\Preferences\{closure}(array, PreferencesFormLegacy)
#6 /vagrant/mediawiki/includes/htmlform/HTMLForm.php(556): HTMLForm->trySubmit()
#7 /vagrant/mediawiki/includes/htmlform/HTMLForm.php(571): HTMLForm->tryAuthorizedSubmit()
#8 /vagrant/mediawiki/includes/specials/SpecialPreferences.php(159): HTMLForm->show()
#9 /vagrant/mediawiki/includes/specialpage/SpecialPage.php(522): SpecialPreferences->execute(NULL)
#10 /vagrant/mediawiki/includes/specialpage/SpecialPageFactory.php(569): SpecialPage->run(NULL)
#11 /vagrant/mediawiki/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#12 /vagrant/mediawiki/includes/MediaWiki.php(863): MediaWiki->performRequest()
#13 /vagrant/mediawiki/includes/MediaWiki.php(524): MediaWiki->main()
#14 /vagrant/mediawiki/index.php(42): MediaWiki->run()
#15 /var/www/w/index.php(5): require(string)
#16 {main}
```