PluginException: The plugin (language_reference) did not specify an instance class. in TypedDataManager.php

I got this error recently when running SimpleTest locally:

Drupal\Component\Plugin\Exception\PluginException: The plugin (language_reference) did not specify an instance class. in Drupal\Core\TypedData\TypedDataManager->createInstance() (line 94 of /Users/mike/shared/htdocs/dm8/core/lib/Drupal/Core/TypedData/TypedDataManager.php).