desktoppackage/configuration: handle loading more gracefully
ConfigurationContainmentAppearance checks whether the containment switching warning needs to be shown or hidden when the value is changed. This also happens when the plugin is loaded, and at that point the stackview does not have a currentItem yet, so the check for whether to hide it fails.
Instead use optional chaining.
Test:
- Right-click desktop, "Desktop and Wallpaper"
- (without) lots of errors (with) one less error