Fix crash on close.

Summary:
We must dissociate the theme manager from the action group and the menu
before these get deleted.

Test Plan: Opening a reader window and closing it; quitting kmail.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12575