Prevent "TypeError: Cannot read property 'Layout' of null" warning on plasmashell startup

Summary:
When starting plasmashell, the following warning was displayed:

file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Panel.qml:83: TypeError: Cannot read property 'Layout' of null

This adds a check whether the containment is available before attempting to use its layout.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2850