check attribute present before trying to delete it

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@16957 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Core/Folder.py

@@ -472,6 +472,7 @@ class Folder(CopyContainer, CMFBTreeFolder, CMFHBTreeFolder, Base, FolderMixIn,
     Remove remaining attributes from previous btree
     and migration
     """
+    if getattr(self, "_tree", None) is not None:
       delattr(self, "_tree")
 
   def _launchCopyObjectToHBTree(self, tag):