Add exception if sonata_type_admin has not association admin

Builder/FormContractor.php

@@ -122,7 +122,9 @@ class FormContractor implements FormContractorInterface
 
         } else if ($type == 'sonata_type_admin') {
 
-            // nothing here ...
+            if (!$fieldDescription->getAssociationAdmin()) {
+                throw new \RuntimeException(sprintf('The current field `%s` is not linked to an admin. Please create one for the target entity : `%s`', $fieldDescription->getName(), $fieldDescription->getTargetEntity()));
+            }
 
         } else if ($type == 'sonata_type_collection') {