On get reference avoid undefined index, instead throw exception informing undefined reference.
This will improve user interaction, making easier understand what is happening.

Running doctrine:fixtures:load instead:
  [ErrorException]
  Notice: Undefined index: database_1_1 in /home/www/eb/platform/vendor/doctrine/data-fixtures/lib/Doctrine/Common/DataFixtures/ReferenceRepository.php line 145

Will appear:
  [Exception]
  Undefined reference database_1_1