Ref KConfig whilst we're using it

Summary:
In current code we would have a KConfigGroup with a dangling KConfig
deleted after the RHS for the group fetch has finished.

BUG: 394534

Test Plan:
Wrote minimal test case of code
It produced a valgrind warning (weirdly didn't crash though)
Modified to correct version
No longer any warnings

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13599