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

Call of FlashMessage->render() if the task object is not valid
was replaced with appropriate code.

Resolves: #73427
Releases: master, 7.6
Change-Id: I2f21e46814d369519756f35b27ee7b546a6b6938
Reviewed-on: https://review.typo3.org/46658


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