kcms/cursortheme: don't crash when the default theme is not installed
The default theme is either selected from "default" or "KDE Classic". If none of them are installed, defaultIndex is invalid, which will cause a crash.
SENTRY: SYSTEMSETTINGS-8D
The default theme is either selected from "default" or "KDE Classic". If none of them are installed, defaultIndex is invalid, which will cause a crash.
SENTRY: SYSTEMSETTINGS-8D