Fix kontact crash on logout.

Summary:
KMail was creating new Akonadi jobs during mainwindow destruction,
due to not testing the bool in GuiActivateEvent.

Same bt in bug 404881, apparently quitting from the akregator tray icon
[which quits all of kontact... separate issue...] gave the same crash.

BUG: 404881
FIXED-IN: 19.04.2

Test Plan: Run kontact, logout. Hello Drkonqi.

Reviewers: mlaurent, winterz

Reviewed By: mlaurent, winterz

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21626