/**
 * Implements hook_entity_field_access().
 *
 * {@inheritdoc}
 *
 * @todo Move this to a form controller so we can hide the field if it has no
 *       options available to it?
 */
function group_entity_field_access($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, FieldItemListInterface $items = NULL) {
  // Can't retrieve an entity from an empty item list.
  if (!isset($items)) {
    return AccessResult::neutral();
  }

  // We only care about the group_roles field when on a form.
  if ($field_definition->getName() != 'group_roles' || $operation !== 'edit') {
    return AccessResult::neutral();
  }

  **// We only care if it is attached to a group content entity.
  if ($items->getEntity()->getEntityTypeId() != 'group_content') {
    return AccessResult::neutral();
  }**

  /** @var DrupalgroupEntityGroupContentInterface $group_content */
  $group_content = $items->getEntity();

  // We only care if the group content entity is a group membership.
  if ($group_content->getContentPlugin()->getPluginId() != 'group_membership') {
    return AccessResult::neutral();
  }

  // Now that we know we're dealing with a group_roles field on a group
  // membership form, we need to check whether the group membership belongs to a
  // group yet. If not, we can't check for access and should always hide it.
  if (!$group = $group_content->getGroup()) {
    return AccessResult::forbidden();
  }

  // Only group administrators should be able to change the membership roles.
  return AccessResult::forbiddenIf(!$group->hasPermission('administer members', $account));
}

// We only care about the the my_test_field field.
      if ($field_definition->getName() != 'my_test_field'

if ($items->getEntity()->getEntityTypeId() == 'group_content')