When a relationship is missing and used in a grouping function, the warning "Warning: Undefined array key XXX in Drupal\views_aggregator\Plugin\views\style\Table->getCellRaw()" is triggered.
Add a check to ensure that warning is not triggered.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I encountered this issue, looks like I have an aggregated table with a sum on a column that has no values.
This patch fixes the warnings. It's just checking isset($result_row->_relationship_entities[$relationship]) before using the value.