[BUGFIX] Check if popup window is there before focusing it

A popup might be blocked by the browser. Check if the window
is really available before putting focus to it to avoid a JS error.

Resolves: #71705
Releases: master
Change-Id: I30d71ac76509c622c4619857e9c19dc9c20c314a
Reviewed-on: https://review.typo3.org/44806


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Valentin Despa <info@vdespa.de>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>