Issue #3508661: Ajax error when adding some paragraph types

src/Cache/MercuryEditorPreviewCacheContext.php

@@ -36,7 +36,7 @@ class MercuryEditorPreviewCacheContext extends RouteNameCacheContext {
    * {@inheritdoc}
    */
   public function getContext() {
-    if ($this->mercuryEditorContext->isPreview()) {
+    if ($this->mercuryEditorContext->isPreview() && $this->mercuryEditorContext->getEntity()) {
       $entity = $this->mercuryEditorContext->getEntity();
       return 'is_mercury_editor_preview.' . $entity->getEntityTypeId() . '.' . $entity->uuid();
     }