[FIX] website_mail: default could be None

addons/website_mail/models/mail_thread.py

@@ -37,5 +37,7 @@ class MailThread(osv.AbstractModel):
     }
 
     def copy(self, cr, uid, id, default=None, context=None):
+        if default is None:
+            default = {}
         default['website_message_ids'] = []
         return super(MailThread, self).copy(cr, uid, id, default=default, context=context)
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