XClass group property store the group id with a comma (,) at the end

For example if you enter the group "XWiki.XWikiAdminGroup" you end up in the database and in $doc.getValue('group') with "XWiki.XWikiAdminGroup," which makes it totally unusable in a script. Note that user property does not seems to have this issue.

Seems to be a bug in the displayer which send two values (the group and an empty one) instead of one but since it's not a multiselect it ends up serialising a list in a unique String property.