TYPO3 Core

Seems you run through the if branch with this code:

if ($configuration['MM']) {
    /** @var $loadDBGroup \TYPO3\CMS\Core\Database\RelationHandler */
    $loadDBGroup = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Database\\RelationHandler');
    $loadDBGroup->start($pageRecord[$column], $configuration['foreign_table'], $configuration['MM'], $uid, 'pages', $configuration);
    $relatedUids = $loadDBGroup->tableArray[$configuration['foreign_table']];

So $loadDBGroup->tableArray[$configuration['foreign_table']] does not return array, which looks more like a configuration error.

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/28561

Comment of Olly Hader:

... "group/db" tables ("allowed" in TCA configuration) was not considered before - thus, the RootlineUtility handling in this regard is broken... This patch would just log away the Core bug... A good inspiration on how to use the RelationHandler is DataHandler, there e.g. look at method "remapListedDBRecords_procDBRefs()"

No progress for a year. Closing this.