Validation of the Flickr Embed Code isn't working

Problem/Motivation

Validation of the Flickr Embed Code / URL isn't working

Steps to reproduce

Create a new Flickr Media Entity, type any random text in the "Flickr URL" field, and click Save.

It creates the new entity without any complaint, but the thumbnail is just the default Flickr logo.

Proposed resolution

The problem is in function validate() in FlickrEmbedCodeConstraintValidator.php, The current code leaves $data as an empty string, but it should be the text from the "Flickr URL" field, for validation:

    $data = '';
    if (is_string($value)) {
      $data = $value;
    }
    elseif ($value instanceof FieldItemInterface) {
      $class = get_class($value);
      $property = $class::mainPropertyName();
      if ($property) {
        $data = $value->{$property};
      }
    }

I don't understand how that is supposed to work, but if I replace it with code copied from the validator from OEMbed:

    $media = $value->getEntity();
    $source = $media->getSource();
    $data = $source->getSourceFieldValue($media);

Then $data is correctly set to the value the user entered, so it can be checked by rest of the validate() function.

Remaining tasks

User interface changes

API changes

Data model changes