Fixing request #5969 Error generated when moving a PBI from the Release Plan back to the Product Backlog

Change-Id: I34c800defc67e214e7025117aee61ffc21374526
Merge commit 'refs/changes/15/2015/5' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fixing request #5969 Error generated when moving a PBI from the Release Plan back to the Product Backlog

Change-Id: Ie462dae8b5387ed0dfdfad622277292621197d9e
This is Tuleap 6.10.99.61

Change-Id: I24f165b12c294dc2453af6e8b965e4fe2ff8aa8c