Fix crash while entering selection mode with Qt6.7
deleteLater() on a non-existing object seems to cause a crash with
Qt6.7. This makes some sense but wasn't the case previously.
I didn't test this yet but it is a harmless change so if anyone can confirm we can IMO merge directly.
This was brought up in !764 (comment 920935), reported in the bug linked below, and the sentry crash reporting page also has a few dozen reports (https://crash-reports.kde.org/organizations/kde/issues/13589/?project=4&query=is%3Aunresolved&referrer=issue-stream&stream_index=1).
Dolphin 24.02 is also affected but we are already past the last bug fix release for it.