Fixed editor drag/drop getting stuck

no issue

- sometimes it was possible for a drop target element to not have a child which would cause an error and cause drag/drop to get stuck with the dragged element sticking to the cursor with no action occurring on mouseup, requiring an `Esc` press but all further drags also being broken
- added a guard so we can handle `element.firstChild` not existing

ghost/admin/lib/koenig-editor/addon/components/koenig-editor.js

@@ -1234,9 +1234,11 @@ export default Component.extend({
             return;
         }
 
-        let cardId = cardElement.firstChild.id;
+        let cardId = cardElement.firstChild?.id;
 
-        return this.componentCards.findBy('destinationElementId', cardId);
+        if (cardId) {
+            return this.componentCards.findBy('destinationElementId', cardId);
+        }
     },
 
     getSectionFromCard(card) {