[BUGFIX] Fix dashboard error when event is not initialized

When trying to add a widget when events not applied caused
the add widget button to throw "undefined array key 'widget'".

This is fixed by removing the url from the affected buttons.
Tests have also been added for "create dashboard", "add widget",
"delete dashboard" and "remove widget".

Resolves: #94396
Releases: master
Change-Id: Ifa64da17c901145016f22a475e655ad475e46c37
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69542


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>