SessionContoller: fix crash when closing session from Konsole KPart

_preventClose has to be set to false right at the end, so as to prevent
crashes if the user selects the "close session" action from the context
menu in a Konsole KPart (e.g. the terminal panel in Dolphin).

Check menu object isn't null before deleting actions from it.

BUG: 429538
(cherry picked from commit 043c424a)