Internal error

[c49422933ba450a48171e65c] /wiki/Newsletter:TestWiki_Admin_Newsletter MWContentSerializationException: The Newsletter content is invalid.

Backtrace:

from /var/www/html/extensions/Newsletter/includes/Content/NewsletterContentHandler.php(72)
#0 /var/www/html/includes/Revision/RevisionStore.php(1237): MediaWiki\Extension\Newsletter\Content\NewsletterContentHandler->unserializeContent()
#1 /var/www/html/includes/Revision/RevisionStore.php(1520): MediaWiki\Revision\RevisionStore->loadSlotContent()
#2 [internal function]: MediaWiki\Revision\RevisionStore->MediaWiki\Revision\{closure}()
#3 /var/www/html/includes/Revision/SlotRecord.php(324): call_user_func()
#4 /var/www/html/includes/Revision/RevisionRecord.php(183): MediaWiki\Revision\SlotRecord->getContent()
#5 /var/www/html/includes/Revision/RenderedRevision.php(229): MediaWiki\Revision\RevisionRecord->getContentOrThrow()
#6 /var/www/html/includes/Revision/RevisionRenderer.php(223): MediaWiki\Revision\RenderedRevision->getSlotParserOutput()
#7 /var/www/html/includes/Revision/RevisionRenderer.php(164): MediaWiki\Revision\RevisionRenderer->combineSlotOutput()
#8 [internal function]: MediaWiki\Revision\RevisionRenderer->MediaWiki\Revision\{closure}()
#9 /var/www/html/includes/Revision/RenderedRevision.php(199): call_user_func()
#10 /var/www/html/includes/poolcounter/PoolWorkArticleView.php(84): MediaWiki\Revision\RenderedRevision->getRevisionParserOutput()
#11 /var/www/html/includes/poolcounter/PoolWorkArticleView.php(69): PoolWorkArticleView->renderRevision()
#12 /var/www/html/includes/poolcounter/PoolCounterWork.php(167): PoolWorkArticleView->doWork()
#13 /var/www/html/includes/page/ParserOutputAccess.php(304): PoolCounterWork->execute()
#14 /var/www/html/includes/page/Article.php(746): MediaWiki\Page\ParserOutputAccess->getParserOutput()
#15 /var/www/html/includes/page/Article.php(550): Article->generateContentOutput()
#16 /var/www/html/includes/actions/ViewAction.php(78): Article->view()
#17 /var/www/html/includes/MediaWiki.php(583): ViewAction->show()
#18 /var/www/html/includes/MediaWiki.php(363): MediaWiki->performAction()
#19 /var/www/html/includes/MediaWiki.php(960): MediaWiki->performRequest()
#20 /var/www/html/includes/MediaWiki.php(613): MediaWiki->main()
#21 /var/www/html/index.php(50): MediaWiki->run()
#22 /var/www/html/index.php(46): wfIndexMain()
#23 {main}