Adding modules that hook into the feed config is not working

When defining the feed custom config will not save. This is being caused by an issue in the Drupal\feeds\Plugin\Field\FieldType\SerializedItem class.

Currently, the Drupal\aggregator\Entity\Feed class uses

$fields['config'] = BaseFieldDefinition::create('feeds_serialized')

If this is simply changed to
$fields['config'] = BaseFieldDefinition::create('map')

This resolves the issue.

We have tested the Download fetcher with rss, opml, and a new JSON parser and are not seeing any issues.

The error happens in Drupal\Core\TypedData\Validation\RecursiveContextualValidator
protected function validateNode(TypedDataInterface $data, $constraints = NULL, $is_root_call = FALSE) {
...
// If the data is a list or complex data, validate the contained list items
// or properties. However, do not recurse if the data is empty.
if (($data instanceof ListInterface || $data instanceof ComplexDataInterface) && !$data->isEmpty()) {
foreach ($data as $name => $property) {
$this->validateNode($property); #this is where the error happens
}
}

Here's the error:
Recoverable fatal error: Argument 1 passed to Drupal\Core\TypedData\Validation\RecursiveContextualValidator::validateNode() must implement interface Drupal\Core\TypedData\TypedDataInterface, array given, called in /datadg/www/drupal/zckb/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php on line 152 and defined in Drupal\Core\TypedData\Validation\RecursiveContextualValidator->validateNode() (line 125 of /datadg/www/drupal/zckb/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php).

Another option to fix the problem if the SerializedItem class is actually needed is to have the propertyDefinitions() method return an empty array().

This issue prevents modules from being added that extend the Feed form when they use the 'config' field to store the settings.