Summary:
qDeleteAll will iterate through containments() however this is
dynamically changing the list we're iterating through.
Particularly bad is deleting the panel might delete the system tray
containment, leading to a double delete on a now dangly pointer.
Test Plan: kquitapp5 plasmashell. No crash
Reviewers: #plasma
Subscribers: plasma-devel
Tags: #plasma
Differential Revision: https://phabricator.kde.org/D1890