[BUGFIX] ManageReferenceIndex: Removed call to deprecated method render()

Call of FlashMessage->render() was replaced with appropriate code.

Resolves: #73420
Releases: master, 7.6
Change-Id: Id6ec612974a1813bf8becd843d25a48b663766aa
Reviewed-on: https://review.typo3.org/46636


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>