##### Impact
Upon deployment of 1.35.0-wmf.39 Flow ends up broken. Each access to a topic cause one occurence of Messages: $key must be a string or an array, and several occurences of extensions/ElectronPdfService/src/ElectronPdfServiceHooks.php:62
PHP Notice: Undefined index: print
The Flow page ends up being broken cause `PHP Notice: Undefined index: print` triggers an exception error.
Both traces seems to relate to building the sidebar
```name=Error message
/srv/mediawiki/php-1.35.0-wmf.39/includes/language/Message.php:261
$key must be a string or an array
```
```lines=12,name=Stack Trace
#0 /srv/mediawiki/php-1.35.0-wmf.39/includes/GlobalFunctions.php(1223): Message->__construct(integer)
#1 /srv/mediawiki/php-1.35.0-wmf.39/includes/context/RequestContext.php(443): wfMessage(integer)
#2 /srv/mediawiki/php-1.35.0-wmf.39/includes/context/ContextSource.php(169): RequestContext->msg(integer)
#3 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/Skin.php(2212): ContextSource->msg(integer)
#4 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/Skin.php(2348): Skin->makeLink(integer, NULL, array)
#5 /srv/mediawiki/php-1.35.0-wmf.39/skins/Vector/includes/VectorTemplate.php(411): Skin->makeListItem(integer, NULL, array)
#6 /srv/mediawiki/php-1.35.0-wmf.39/skins/Vector/includes/VectorTemplate.php(325): VectorTemplate->getMenuData(string, array, integer)
#7 /srv/mediawiki/php-1.35.0-wmf.39/skins/Vector/includes/VectorTemplate.php(171): VectorTemplate->buildSidebar()
#8 /srv/mediawiki/php-1.35.0-wmf.39/skins/Vector/includes/VectorTemplate.php(199): VectorTemplate->getSkinData()
#9 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/SkinTemplate.php(127): VectorTemplate->execute()
#10 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/SkinTemplate.php(141): SkinTemplate->generateHTML()
#11 /srv/mediawiki/php-1.35.0-wmf.39/includes/OutputPage.php(2616): SkinTemplate->outputPage()
#12 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(993): OutputPage->output(boolean)
#13 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(1006): MediaWiki->{closure}()
#14 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(543): MediaWiki->main()
#15 /srv/mediawiki/php-1.35.0-wmf.39/index.php(53): MediaWiki->run()
#16 /srv/mediawiki/php-1.35.0-wmf.39/index.php(46): wfIndexMain()
#17 /srv/mediawiki/w/index.php(3): require(string)
#18 {main}
```
```name=Error message
/srv/mediawiki/php-1.35.0-wmf.39/extensions/ElectronPdfService/src/ElectronPdfServiceHooks.php:62
PHP Notice: Undefined index: print
```
```lines=12,name=Stack Trace
#0 /srv/mediawiki/php-1.35.0-wmf.39/extensions/ElectronPdfService/src/ElectronPdfServiceHooks.php(62): MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 /srv/mediawiki/php-1.35.0-wmf.39/includes/HookContainer/HookContainer.php(320): ElectronPdfServiceHooks::onSidebarBeforeOutput(SkinVector, array)
#2 /srv/mediawiki/php-1.35.0-wmf.39/includes/HookContainer/HookContainer.php(131): MediaWiki\HookContainer\HookContainer->callLegacyHook(string, array, array, array)
#3 /srv/mediawiki/php-1.35.0-wmf.39/includes/HookContainer/HookRunner.php(3490): MediaWiki\HookContainer\HookContainer->run(string, array, array)
#4 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/Skin.php(1649): MediaWiki\HookContainer\HookRunner->onSidebarBeforeOutput(SkinVector, array)
#5 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/SkinTemplate.php(432): Skin->buildSidebar()
#6 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/SkinTemplate.php(125): SkinTemplate->prepareQuickTemplate()
#7 /srv/mediawiki/php-1.35.0-wmf.39/includes/skins/SkinTemplate.php(141): SkinTemplate->generateHTML()
#8 /srv/mediawiki/php-1.35.0-wmf.39/includes/OutputPage.php(2616): SkinTemplate->outputPage()
#9 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(993): OutputPage->output(boolean)
#10 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(1006): MediaWiki->{closure}()
#11 /srv/mediawiki/php-1.35.0-wmf.39/includes/MediaWiki.php(543): MediaWiki->main()
#12 /srv/mediawiki/php-1.35.0-wmf.39/index.php(53): MediaWiki->run()
#13 /srv/mediawiki/php-1.35.0-wmf.39/index.php(46): wfIndexMain()
#14 /srv/mediawiki/w/index.php(3): require(string)
#15 {main}
```
##### Notes