group displayer doesn't work in view, preview mode and export

Hello,
For my Xwiki application, I created my Class with Group properties), a Sheet and Template.
My field working well in edit mode but are wrong displayed in others mode

• in preview and PDF/ODT export file, I obtain "Unknown display mode." instead of group name
• in view mode, I obtain empty avatar and this code for each group I add:

  ${escapetool.xml($groupTitle)}
  ${escapetool.xml($groupReference.name)}
  (${escapetool.xml($groupReference.wikiReference.name)}) 
  

Thxs.