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

Calls of FlashMessage->render() are replaced with appropriate code.

Resolves: #72439
Releases: master, 7.6
Change-Id: Ia7288fe06baa10c4710f95876488911f1d3d1691
Reviewed-on: https://review.typo3.org/45440


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>