[BUGFIX] Fix regression in AbstractTreeView

Fix a wrong count which has been implemented with #75645.

Resolves: #76632
Releases: master
Change-Id: I6d3fde6c46153baa8b8eb549ca3781aa455d2cdf
Reviewed-on: https://review.typo3.org/48576


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>