[TASK] Fix possible null pointer exception

AbstractTreeView defines the current "scriptUrl",
to build links in the backend. Since this class
may also be called via CLI, e.g. by EXT:impexp,
missing `instanceof` checks are added to prevent
a null pointer exception.

Resolves: #94947
Related: #93158
Releases: master
Change-Id: I795effd17d42944ae0ab32b1ab10cd632ca7c31e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70689


Tested-by: core-ci <typo3@b13.com>
Tested-by: Alexander Nitsche <typo3@alexandernitsche.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Alexander Nitsche <typo3@alexandernitsche.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>