avoid that the main context can be #poped

Summary:
avoids that the main context can be #poped
+ ensure we abort the switching loop if we arrive at it

Test Plan: make && make test

Reviewers: dhaumann, vkrause, nibags

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D15835