Serialization error when switching between poll form and poll results as an anonymous user on PHP 7.4

Problem/Motivation

When an anonymous user navigates to a poll, clicks the "View results" button and then goes back to the poll form by clicking the "View poll" button, a serialization error occurs on PHP 7.4.

Notice: unserialize(): Error at offset 4261 of 12438 bytes in Drupal\Component\Serialization\PhpSerialize::decode() (line 21 of core/lib/Drupal/Component/Serialization/PhpSerialize.php).

The error only occurs as an anonymous user, because when logged in as an authenticated user, the "View poll" button doesn't appear, which is a different bug: #3178172: Button "View poll" is missing for authenticated users. But even when changing code to make the button appear for authenticated users, there is still no serialization error for them.

I noticed this error while I was working on #2909811: Anonymous users - Multiple voting from one IP and there it prevents the tests from passing on PHP 7.4.

Steps to reproduce

1. Set the PHP version to 7.4.
2. Create a poll that allows anonymous votes and allows to view the results before voting.
3. As an anonymous user, go to the poll. The poll form gets displayed.
4. Click the "View results" button. The poll vote results get displayed.
5. Click the "View poll" button.

The poll form gets displayed again, but there also gets a serialization error logged. When the user has javascript disabled and the site is configured to display errors, the error is also displayed on the page.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

A patch with tests to demonstrate the issue, will follow.