If StackView is empty, currentItem will be null and we would break all over the place.
This can happen when you have a wallpaper without config, so "sourceFile" would stay empty and thus onSourceFileChanged wouldn't be triggered and we wouldn't load anything into our StackView.

BUG: 360862
FIXED-IN: 5.8.6

Switching between wallpapers still works (minus the usual glitches of default values not being applied but that's a different story) and applying settings. I can now switch between Hunyango and Haenau no problem.

Ideally this dialog was refactored to use onActivated instead of onCurrentIndexChanged and Component.onCompleted to populate itself etc etc but when I did something similar to applet config dialog, it broke various places relying on the fact that values change *after* creation, so let's go with this least invasive patch, for 5.8 series anyway.