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location of panel configuration file (lost my settings)

I'm running KDE on Arch.  Here's some system info:

plasmashell --version: plasmashell 5.12.5
dolphin --version: dolphin 18.04.0
Linux 4.16.6-1-ARCH #1 SMP PREEMPT Mon Apr 30 12:30:03 UTC 2018 x86_64 GNU/Linux

I was using my computer today and everything was normal. I came back, unlocked the screen saver, and when my desktop appeared, my panel was gone. I had done a lot of customizations to my panel. I'd like to restore those from a backup.

Which file contains the panel settings (full path and filename so I can find it in my backup)?