Fix warning

Summary:
.../kirigami.2/templates/AbstractApplicationHeader.qml:113: TypeError: Cannot read property 'globalToolBarItem' of null
.../kirigami.2/templates/AbstractApplicationHeader.qml:114: TypeError: Cannot read property 'globalToolBarItem' of null

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D17268