[BUGFIX] Properly handle unknown link type

In case an invalid link type is used in
a link field, the link browser does no
longer throw an exception but instead
displays a flash message with the
corresponding information.

Additionally, an undefined array key access
is resolved, which might happen e.g. in
case such invalid link is being analyzed
by link validator.

Resolves: #102804
Releases: main, 12.4
Change-Id: I5ad35ff46360cc4b27556b8e2c75da69ecbe76d9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82387


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>