Created attachment 179120 [details]
"coredumpctl gdb" result; removed 60000 entries to fit the file size limit; the looped entries were the same for 60k lines.

SUMMARY
Changing to a third-party Plasma Theme and then Adding a widget to a panel or desktop crashes plasmashell

STEPS TO REPRODUCE
1. Install and Apply a third-party Plasma Theme through System Settings
2. On Desktop enter Edit Mode and then enter Add or Manage Widgets
3. Drag & Drop a Widget on a Panel or the Desktop

OBSERVED RESULT
Cursor shows icon "Action is not allowed" or "Action cannot be done" (the red X).
Plasmashell freezes for a few seconds, crashes and then replaces itself.

EXPECTED RESULT
Normally add a Widget to a Panel or the Desktop

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 6.3.2
KDE Frameworks Version: 6.11.0
Qt Version: 6.8.2
Kernel: 6.12.17-1-lts

ADDITIONAL INFORMATION
I brought this issue up on r/kde on Reddit.
Adding a link to the post.
https://www.reddit.com/r/kde/comments/1j2u50x/are_there_plasma_themes_specifically_for_plasma_6/