Warning: array_flip(): Can only flip STRING and INTEGER values!

I'm digging through some old bug reports.

I see that the double usage of array_flip() can be avoided. Instead, I think it's better to use array_search() to look for the entry from the array to remove. I still don't how the bug could be reproduced though.

Upon further looking at the stack trace, the array_flip() error did not happen in \Drupal\feeds\Entity\FeedType::__sleep(), but in \Drupal\Core\Entity\EntityStorageBase::loadMultiple() instead:

  /**
   * {@inheritdoc}
   */
  public function loadMultiple(?array $ids = NULL) {
    $entities = [];
    $preloaded_entities = [];

    // Create a new variable which is either a prepared version of the $ids
    // array for later comparison with the entity cache, or FALSE if no $ids
    // were passed. The $ids array is reduced as items are loaded from cache,
    // and we need to know if it is empty for this reason to avoid querying the
    // database when all requested entities are loaded from cache.
    $flipped_ids = $ids ? array_flip($ids) : FALSE;

The error looks like to be triggered by either an entity reference field or a file field. While theoritically this could come from a bug in Feeds, it is not clear if this is the case. Since this issue is over 3 years old now, I think I'm just going to close it. Feel free to reopen if you still encounter this issue and if you can provide the steps to reproduce the issue.

Note: I did merge a change for FeedType::__sleep(), because the double usage of array_flip() was not needed.