Ran into this little problem this morning, with desktop settings. (KDE Neon dev stable.)
Please see image. https://drive.google.com/open?id=0B3b7vABNYxgYd3dYbFpHdEFiY3c

Similar problem for me.
LinuxMint with KDE desktop v 18, using KDE plasma v5.6.5, KDE frameworks 5.23.0, Qt version 5.5.1
When I right-click on the desktop and click on Desktop Settings, then choose *Get New Wallpapers I am presented with an empty window. In the left-lower corner is the phrase "Some categories are missing"