erp5_project: avoid can't access error when get contents

if such error occurs, this listfield show "You selected an item that was not in list" error message when save
7 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:16:40

passed ERP5.PerformanceTest-Master

00:32:59

failed ERP5.UnitTest-Master

02:14:06

passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:37

passed SlapOS.Eggs.UnitTest-Master.Python3

01:01:33

passed SlapOS.Eggs.UnitTest-Master.Python2

01:20:15

passed SlapOS.Eggs.UnitTest-Master.Python3

01:13:10