Issue #3439042 by manuel.adan: Cannot create holders for content entities without bundles

src/Entity/EntityHolder.php

@@ -161,7 +161,8 @@ class EntityHolder extends ConfigEntityBase implements EntityHolderInterface {
       throw new \InvalidArgumentException('Entity type mismatch.');
     }
 
-    if ($this->getHeldEntityBundle() != $entity->bundle()) {
+    $expected_bundle = $this->getHeldEntityBundle();
+    if (!empty($expected_bundle) && $expected_bundle != $entity->bundle()) {
       throw new \InvalidArgumentException('Entity bundle mismatch.');
     }