Owning user deletion and errors on default formatters (and others)

[DiegoPino]

What?

If a SBF on any entity (custom or not) is present on an Object owned by a no longer existing user, an uncaught error happens in this form

The website encountered an unexpected error. Please try again later.
Error: Call to a member function id() on null in Drupal\strawberryfield\Plugin\Field\FieldFormatter\StrawberryDefaultFormatter->viewElements() (line 69 of modules/contrib/strawberryfield/src/Plugin/Field/FieldFormatter/StrawberryDefaultFormatter.php).

The longer story

Drupal handles user removal v/s entities belonging to them as a batch action performed over those entities on the User module. Because we have our own custom entities (provided by every module) we either need to do that too OR we need to allow an Admin to correct ownership after the fact without a whitescreenofdeath

Extra info

Some entities deal with this by exposing the user as anonymous instead of failing. e.g Comment one does this:

/**
   * {@inheritdoc}
   */
  public function getOwner() {
    $user = $this->get('uid')->entity;
    if (!$user || $user->isAnonymous()) {
      $user = User::getAnonymousUser();
      $user->name = $this->getAuthorName();
      $user->homepage = $this->getHomepage();
    }
    return $user;
  }

But e.g for AMI Set Entities we do it wrong

  /**
   * {@inheritdoc}
   */
  public function getOwner() {
    return $this->get('user_id')->entity;
  }

@alliomeria @aksm

[DiegoPino]

partially solved via a9bbb81
We need to check any other custom entity for the same code. Are Metadata Displays dealing with this correctly?
My concern is that just changing the user ownership (via the return) might lead to exposing a private Entity to the public?

@aksm I merged it for now but we need to be sure that returning the Anonymous user is not dangerous, maybe we should do the opposite, return the Admin one?