[TASK] Fix missing variable initialization in AbstractTreeView

https://review.typo3.org/49750 introduced code that lacks proper
variable initialization.
Get this straight.

Resolves: #79620
Releases: master, 7.6
Change-Id: I1b5735807f7f0fc476a5370f23d6b7d4654e4599
Reviewed-on: https://review.typo3.org/51527


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>