[applets/showdesktop] Fix errors when asynchronous loader hasn't initialized yet
/usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:161: TypeError: Cannot read property 'active' of null
/usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:39: TypeError: Cannot read property 'active' of null
/usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:32: TypeError: Cannot read property 'active' of null