Issue #3309747: Support CKEditor4 and CKEditor5

Closes #3309747

src/Access/EmbedButtonEditorAccessCheck.php

@@ -78,20 +78,29 @@ class EmbedButtonEditorAccessCheck implements AccessInterface {
    *   currently only capable of detecting buttons used by CKEditor.
    */
   protected function checkButtonEditorAccess(EmbedButtonInterface $embed_button, EditorInterface $editor) {
-    if ($editor->getEditor() !== 'ckeditor') {
+    if (!in_array($editor->getEditor(), ['ckeditor', 'ckeditor5'], TRUE)) {
       throw new HttpException(500, 'Currently, only CKEditor is supported.');
     }
 
     $has_button = FALSE;
     $settings = $editor->getSettings();
-    foreach ($settings['toolbar']['rows'] as $row) {
-      foreach ($row as $group) {
-        if (in_array($embed_button->id(), $group['items'])) {
-          $has_button = TRUE;
-          break 2;
+    if ($editor->getEditor() === 'ckeditor') {
+      foreach ($settings['toolbar']['rows'] as $row) {
+        foreach ($row as $group) {
+          if (in_array($embed_button->id(), $group['items'])) {
+            $has_button = TRUE;
+            break 2;
+          }
         }
       }
     }
+    elseif ($editor->getEditor() === 'ckeditor5') {
+      // The schema for CKEditor5 has changed, therefore we need to check for
+      // the toolbar items differently.
+      if ($settings['toolbar']['items'] && in_array($embed_button->id(), $settings['toolbar']['items'])) {
+        $has_button = TRUE;
+      }
+    }
 
     return AccessResult::allowedIf($has_button)
       ->addCacheableDependency($embed_button)