[BUGFIX] JS: Init SelectTreeElement after document.ready

Add document.ready, so SelectTree is initialized even
with slow internet connection.

Resolves: #81891
Releases: master, 8.7
Change-Id: I5489d37821107310f99fbccffc00fd183e2e3786
Reviewed-on: https://review.typo3.org/54310


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>