AbstractApplicationItem: Fix copy-pasta mistake
It used to cause error logs like this:
file:///usr/lib/qml/org/kde/kirigami.2/AbstractApplicationItem.qml:93: TypeError: Cannot read property 'position' of null
file:///usr/lib/qml/org/kde/kirigami.2/AbstractApplicationItem.qml:92: TypeError: Cannot read property 'background' of null