Issue #3263496 by mupsi: Fix error adding element to library from default layout form by supporting Display and Entity contexts for default and overridden layouts

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• File an issue
• Fix bugs in code
• Merge request
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
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• Full testing and approval
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src/Form/AddSectionToLibraryForm.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\section_library\Form;
 
+use Drupal\Component\Plugin\Exception\ContextException;
 use Drupal\Core\Ajax\AjaxFormHelperTrait;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
@@ -146,7 +147,19 @@ class AddSectionToLibraryForm extends FormBase {
   public function submitForm(array &$form, FormStateInterface $form_state) {
     $current_section = $this->sectionStorage->getSection($this->delta);
     $deep_cloned_section = $this->deepCloneSection($current_section);
-    $layout_entity = $this->sectionStorage->getContextValue('entity');
+
+    $layout_entity = NULL;
+    foreach (['entity', 'display'] as $name) {
+      try {
+        $layout_entity = $this->sectionStorage->getContextValue($name);
+      }
+      catch (ContextException $e) {
+        // Let's try again.
+      }
+    }
+    if (!$layout_entity) {
+      throw new ContextException('', 0, $e);
+    }
 
     $entity_values = [
       'label' => $form_state->getValue('label'),