I recently noticed that I have this problem too!
When I right click on desktop → click on Add Widgets... option, Nothing happens! The Widgets list doesn’t open.
Also when I tried through Right click on panel → Click on Add Widgets... option, again nothing happens.
How to fix this problem?
Is there any way to open Widgets list using terminal?
My system information:
$ uname -a
Linux milad-pc 5.4.164-1-MANJARO #1 SMP PREEMPT Wed Dec 8 09:53:07 UTC 2021 x86_64 GNU/Linux
I don’t know whether this will help, but what you could try is run the following command in a terminal window, although best would be to do it from a tty while completely logged out of Plasma… 
kbuildsycoca5 --noincremental
This should restore your configuration. If it doesn’t make any difference, then you’ve got other issues.
I didn’t find the reason of this problem but I got rid of it by a simple reboot:
$ shutdown -r now
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