Due to T95580: Flow data missing on Wikimedia production wikis there can be missing content (mainly/solely on test wikis). This needs to be handled when dumping.
ArielGlenn had this happen on testwiki (T89398: Add Flow to database dumps):
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}