Michael Vanetta
--- a/core/tests/Drupal/KernelTests/Core/Form/FormAfterBuildTest.php 0 → 100644
+++ b/core/tests/Drupal/KernelTests/Core/Form/FormAfterBuildTest.php 0 → 100644
+
+  /**
+   * Tests a form with an #after_build that adds an element with no '#parents'.
+   */
+  public function testUndefinedParentsInAfterBuild() {
+    $form_state = new FormState();
+    $form_builder = $this->container->get('form_builder');
+    $error_message = NULL;
+    try {
+      $form_builder->submitForm($this, $form_state);
+    }
+    catch (\Throwable $e) {
+      $error_message = $e->getMessage();
+    }
+
+    $this->assertTrue(empty($error_message), "The following error occurred during the form submission: {$error_message}");
-    $error_message = NULL;
-    try {
-      $form_builder->submitForm($this, $form_state);
-    }
-    catch (\Throwable $e) {
-      $error_message = $e->getMessage();
-    }
-
-    $this->assertTrue(empty($error_message), "The following error occurred during the form submission: {$error_message}");
+      $this->assertNull($form_builder->submitForm($this, $form_state), "Form submitted without triggering an exception");
--- a/core/modules/system/tests/modules/form_test/form_test.module
+++ b/core/modules/system/tests/modules/form_test/form_test.module
+/**
+ * Implements hook_form_FORM_ID_alter().
+ */
+function form_test_form_form_after_build_test_alter(&$form, FormStateInterface $form_state, $form_id) {
+  $form['#after_build'][] = '_form_test_after_build';
+}
+
+/**
+ * After build callback to prove !empty check on #parents is necessary.
+ *
+ * @see https://www.drupal.org/project/drupal/issues/3027240
+ */
+function _form_test_after_build($element, FormStateInterface $form_state) {
+  // Add a new element to the form that doesn't have #parents set.
+  $element['after_build_item'] = ['#markup' => 'after_build_item'];
+  return $element;
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
   * {@inheritdoc}
   */
  public function getError(array $element) {
-    if ($errors = $this->getErrors()) {
+    if (isset($element['#parents']) && $errors = $this->getErrors()) {
      $parents = [];
      foreach ($element['#parents'] as $parent) {