Due to {T95580} there can be missing content (mainly/solely on test wikis). This needs to be handled when dumping.
ArielGlenn had this happen on testwiki (T89398):
```
command /usr/bin/php5 -q /srv/mediawiki/multiversion/MWScript.php extensions/Flow/maintenance/dumpBackup.php --wiki=testwiki --current --report=1000 --output=bzip2:/mnt/data/xmldatadumps/public/testwiki/20160601/testwiki-20160601-flow.xml.bz2 (21805) started...
[effb0c25c66162949d5acb27] [no req] Flow\Exception\InvalidDataException from line 366 of /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Model/AbstractRevision.php: Failed to load the content
Backtrace:
#0 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(395): Flow\Mode\\AbstractRevision->getContent(string)
#1 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(360): Flow\Dump\Exporter->formatRevision(Flow\Model\PostRevision)
#2 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(255): Flow\Dump\Exporter->formatRevisions(Flow\Model\PostRevision)
#3 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(213): Flow\Dump\Exporter->formatPost(Flow\Model\PostRevision)
#4 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(196): Flow\Dump\Exporter->formatTopic(Flow\Model\PostRevision)
#5 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/includes/Dump/Exporter.php(174): Flow\Dump\Exporter->formatWorkflow(Flow\Model\Workflow, Flow\Search\Iterators\HeaderIterator, Flow\Search\Iterators\TopicIterator)
#6 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/maintenance/dumpBackup.php(85): Flow\Dump\Exporter->dump(BatchRowIterator)
#7 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/maintenance/dumpBackup.php(58): FlowDumpBackup->dump(integer)
#8 /srv/mediawiki/php-1.28.0-wmf.7/maintenance/doMaintenance.php(103): FlowDumpBackup->execute()
#9 /srv/mediawiki/php-1.28.0-wmf.7/extensions/Flow/maintenance/dumpBackup.php(124): require_once(string)
#10 /srv/mediawiki/multiversion/MWScript.php(97): require_once(string)
#11 {main}
```