I got this PHP Notice error while using MB Relationship together with FacetWP.
With FacetWP activated the normalize() method is getting called twice, resulting in this issue.
On line 79 $direction is set from the $args['from']:
$direction = isset( $args['from'] ) ? 'from' : 'to';
Which is then used on line 84:
$args['items'] = $this->get_ids( $args[ $direction ], $args['id_field'] );
On line 87 the original parameter is cleared:
unset( $args[ $direction ] );
Then the second time around the original 'from' parameter does not exist anymore and the notice is triggered.
Perhaps the normalize_args() method could check if it has already run?