LogicException: The database connection is not serializable.

Problem/Motivation

File fields on core contact forms trigger the error:

LogicException: The database connection is not serializable.

This seems to happen when a user with a role that is tracked by Pardot and a form that is mapped.

The file upload field uses an AJAX request to upload the file to temporary storage. The file upload actually fails and the message is logged. The form can be submitted, but the file is never attached to the form.

Disabling Pardot on either the form or for the user role fixes the problem.

This error seems to occur in a lot of situations, and the solution seems to be adding DependencySerializationTrait to the appropriate file.

I added this to \Drupal\pardot\FormOperations, and the file field starts working again.

Steps to reproduce

1. Configure a contact form with a core file field.
2. Configure Pardot for the form and the user role.
3. Attempt to upload a file to the form.

Upload fails.

Proposed resolution

Add Drupal\Core\DependencyInjection\DependencySerializationTrait; to FormOperations.php.

Remaining tasks

Maybe additional research should happen to determine why the database needs to be serialized, and determine if this is the most appropriate solution.