Description:

It would appear that after editting a MassMessage delivery list description (a page with such content model) its cache is not purged, causing the whole page to display as empty.

Steps to reproduce:

• Go to e.g. https://meta.wikimedia.org/wiki/User:MarcoAurelio/MM-bugtest and hit edit: https://meta.wikimedia.org/wiki/Special:EditMassMessageList/User:MarcoAurelio/MM-bugtest
• Add something to the Description field, then save.

Expected results:

• The page is updated and the updated content is displayed.

Actual results:

• The page content vanishes and can only be restored by manually purging the page cache (?action=purge)

Screenshots:

Before editting	After editting

Software information:

• Site: https://meta.wikimedia.org
• MediaWiki: 1.39.0-wmf.6 (d88a0c4)
• MassMessage: 0.4.0 (6078343)