[BUGFIX] Usage of undefined variables in ShortcutToolbarItem

Fix a undefined variable and a wrong variable assignment in the
ShortcurtToolbarItem.

Resolves: #55998
Releases: 6.2, 6.1, 6.0
Change-Id: I0f0ebcc846a9aa56edd05e384d62aad8f0a5b05b
Reviewed-on: https://review.typo3.org/27662
Reviewed-by: Markus Klein
Tested-by: Markus Klein