Fix DependentDropdownTest::testDependentDropdown in ajax_example

Problem/Motivation

There was 1 error:

1) Drupal\Tests\ajax_example\FunctionalJavascript\DependentDropdownTest::testDependentDropdown
WebDriver\Exception\CurlExec: Curl error thrown for http POST to http://chromedriver-jenkins-drupal-contrib-662366:9515/session/9f58b803d...

Retries: 10 and last result:
{"sessionId":"9f58b803de9b2a4aabcb2995ff9fc2b6","status":10,"value":{"message":"stale element reference: element is not attached to the page document\n (Session info: headless chrome=106.0.5249.103)\n (Driver info: chromedriver=106.0.5249.61 (511755355844955cd3e264779baf0dd38212a4d0-refs/branch-heads/5249@{#569}),platform=Linux 4.9.0-0.bpo.6-amd64 x86_64)"}}

See https://www.drupal.org/pift-ci-job/2596914

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes