Avoid crash if kicker plugin is used outside plasma context
Summary:
Code currently gets the applet interface object passed in explicitly,
from this it gets the configuration via the metaobject system
We always guard the appletConfig not being available, but never the
appletInterface, resulting in crashes if used from qmlscene or any
potential tests.
Test Plan: Used from qmlscene
Reviewers: Plasma, apol, hein
Reviewed By: Plasma, apol, hein
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D22818