InlineEntityFormSimple can throw a warning (PHP8) that fails any FunctionalTest with an empty inline entity form.

Problem/Motivation

Since upgrading to PHP8, InlineEntityForm::extractFormValues() can trigger a warning when there is no form element for one of the items. I'm not entirely sure how that happens, it might be access control, it might be that the item has been removed from the form. This is a problem, because now any FunctionalTest that uses an inline entity form, where you don't set the value of the field counts as a failure.

PHPUnit 9.5.13 by Sebastian Bergmann and contributors.

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Testing Drupal\Tests\ck_contacts\Functional\ContactAddTypeInterfaceTest
E                                                                   1 / 1 (100%)

Time: 00:13.500, Memory: 6.00 MB

There was 1 error:

1) Drupal\Tests\ck_contacts\Functional\ContactAddTypeInterfaceTest::testAddTestAttorneyRoleToIndividual
Exception: Warning: Trying to access array offset on value of type null
Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormSimple->extractFormValues()() (Line: 140)


/var/www/html/web/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php:49
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:204
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:153
/var/www/html/vendor/guzzlehttp/promises/src/TaskQueue.php:48
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:248
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:224
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:269
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:226
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:62
/var/www/html/vendor/guzzlehttp/guzzle/src/Client.php:182
/var/www/html/web/core/tests/Drupal/Tests/DrupalTestBrowser.php:137
/var/www/html/vendor/symfony/browser-kit/Client.php:404
/var/www/html/vendor/symfony/browser-kit/Client.php:324
/var/www/html/vendor/friends-of-behat/mink-browserkit-driver/src/BrowserKitDriver.php:719
/var/www/html/vendor/friends-of-behat/mink-browserkit-driver/src/BrowserKitDriver.php:494
/var/www/html/vendor/behat/mink/src/Element/NodeElement.php:153
/var/www/html/vendor/behat/mink/src/Element/NodeElement.php:161
/var/www/html/vendor/behat/mink/src/Element/TraversableElement.php:115
/var/www/html/web/modules/custom/contacts/tests/src/Functional/ContactAddTypeInterfaceTest.php:69
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestResult.php:726

ERRORS!
Tests: 1, Assertions: 14, Errors: 1.

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The code causing the warning is here:

    foreach ($items as $delta => $value) {
      $element = NestedArray::getValue($form, [$field_name, 'widget', $delta]);
      /** @var \Drupal\Core\Entity\EntityInterface $entity */
      $entity = $element['inline_entity_form']['#entity'];
      $weight = isset($submitted_values[$delta]['_weight']) ? $submitted_values[$delta]['_weight'] : 0;
      $values[$weight] = ['entity' => $entity];
    }

Proposed resolution

Check whether element exists on line 138 before trying to access its properties.