Using the $displayMainMenu variable breaks the content menu buttons

Steps to reproduce:

• copy the template layoutExtraVars.vm
• hide the main menu:

  #set($displayMainMenu = false)
  

• attach layoutExtraVars.vm to the default skin XWiki.DefaultSkin
• open http://localhost:8080/xwiki/bin/view/Sandbox/
  Result:
• on 7.2 when clicking the edit link or the more link you get an error
• on 6.4.5 you get some velocity code on the wiki header