After updating to plasma 5.12.0, kwin's switcher 'Thumbnail Grid' doesn't show anymore in the options (systemsettings5 -> Window Management -> Task Switcher -> Visualization). I can see the following error: Command ' "plasmapkg2 -t windowswitcher -i /tmp/thumbnail_grid-v4.zip" ' failed with code 4 QXcbConnection: XCB error: 3 (BadWindow), sequence: 6718, resource id: 31483117, major code: 40 (TranslateCoords), minor code: 0 If I delete the file $HOME/.local/share/kwin/tabbox/kpluginindex.json then it shows up again in the options, but doesn't work when set up as the window switcher effect.
Please report to your distribution.