Session manager: empty session list

Summary: When opening the session manager, the session list is empty although the menu "sessions" >"quick open session" shows the complete list.

Test Plan:
The "Behaviour on Application Startup" is "Start new session".

Open the session manager through the menu or a shortcut, all the sessions available in "quick open session" are also available.

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, kwrite-devel, #kate

Tags: #kate

Differential Revision: https://phabricator.kde.org/D27719