Created attachment 149491 [details]
Video of the panel widget bug. (Ignore the graphical glitches, as my *real* panel is set to autohide.) This bug can occur on a fresh install too, I've reproduced it in the KDE Neon Testing image too.

SUMMARY
Adding an application to Plasma's panel as a widget will corrupt/destroy the layout of all widgets on the panel, on restarting Plasmashell or logging-out and in, rendering it unusable until the panel is deleted. (A video of the bug was attached as well. The bug will be reproduced on the top panel, I don't want to nuke my working panel again...)

STEPS TO REPRODUCE (WARNING: THIS WILL DESTROY YOUR PANEL.)
1. Open Kickoff.
2. Right click on any application.
3. Select 'Add to Panel (Widget)'
4. Restart Plasmashell by logging out and back in (or running `kquitapp5 plasmashell && kstart5 plasmashell > /dev/null 2>&1`)
5. Watch Plasma's panel load corrupted.

OBSERVED RESULT
Plasma's panel is corrupted; all widgets overlap one another to one corner (I'm not sure if the direction is affected by the alignment settings).

EXPECTED RESULT
Widgets on Plasma's panel show normally.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Leap 15.3
KDE Plasma Version: 5.24.90
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.2
Kernel Version: 5.3.18-150300.59.68-default (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-3320M CPU @ 2.60GHz
Memory: 7.6 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000
Manufacturer: LENOVO
Product Name: 2347G2U
System Version: ThinkPad T430

ADDITIONAL INFORMATION
I'm confident that this is a regression in the Plasma 5.25 Beta (5.24.90) as it worked normally in 5.24.5. I've just upgraded yesterday (missed the Beta Testing day...) and immediately after logging back in, this error occurred. I can't remove the widget easily as I need to 'hover' over it first; that is impossible as all widgets overlap one another. I cannot move the widgets on the panel either, so I just removed the panel and started from scratch.