request #6592: Fix fatal error when deleting a Gerrit migration

Change-Id: Ib85e03ac2893db6c8a723e532201ffad431bda90
Merge commit 'refs/changes/16/2216/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #6592: Fix fatal error when deleting a Gerrit migration

Change-Id: I240d0815694fb40c385ac8a46185ce84278cf527
Tuleap 6.11.99.46

Change-Id: I88d6366cf4b5ce77c2f6b8fa393207dff112620e