Why I'm getting theme related error when trying to add external JS to form? I'm trying to add Google Map API to node form:

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mm_form_node_foo_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id)
{
    $form['#attached']['library'][] = 'google/maps';
}


Library config (mm.libraries.yml in module root directory):

google.maps:
  js:
    https://maps.googleapis.com/maps/api/js?key=API_KEY&amp;libraries=places&amp;callback=initMap: {type: external, attributes: { defer: true, async: true, data-test: map-link } } }


and the error is:


  User warning: The following theme is missing from the file system:
  google in drupal_get_filename() (line 234 of
  core/includes/bootstrap.inc).
You probably need to add the module/theme name before the attached library:

$form['#attached']['library'][] = 'mm/google.maps';


Otherwise drupal is looking for google as your module/theme name.