Fix Plasmashell crash when opening config

[zhimsel]

Fixes https://github.com/kotelnik/plasma-applet-thermal-monitor/issues/67

Written by /u/DGLS575 on Reddit: https://www.reddit.com/r/kdeneon/comments/gkyuij/attempting_to_configure_thermal_monitor_widget/ft3gsvq/

This is a direct patch from the above author. Feel free to modify it as you see fit. I've already tested this and it does fix the crash.

This has been (hopefully temporarily) patched in the Archlinux package:https://aur.archlinux.org/packages/plasma5-applets-thermal-monitor-git