Reduce rare AttributeError on Skins Tool.

Thanks to Vincent Pelletier for his help on this bug fix.
7 jobs for master in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:08:28

passed ERP5.PerformanceTest-Master

00:26:56

passed ERP5.UnitTest-Master

01:28:52

failed ERP5.UnitTest-Zope2

02:39:03

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:14

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:16

passed Wendelin.UnitTest-Master

00:38:50