SUMMARY
The laptop repeatedly crashes when connected to the external monitor. Did not do so in the previous version of Kubuntu.

STEPS TO REPRODUCE
1. Go to 'System Settings' → Applications style → change Global theme. 

2. Alternately, Put the laptop to 'Sleep' when connected to the external monitor. It said once the lock screen is broken. Now I don't see the message, it just freezes and a forced shutdown is needed.

3. The only way to resolve the freeze crash is a reboot.

OBSERVED RESULT
Crash. 

EXPECTED RESULT
To work like the previous version.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: plasmashell (5.21.4)
(available in About System)
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
Graphics Platform: Wayland

ADDITIONAL INFORMATION
Application: Plasma (plasmashell), signal: Aborted
 
[KCrash Handler]
#4 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:49
#5 0x00007fd125243864 in __GI_abort () at abort.c:79
#6 0x00007fd1256b5c43 in QMessageLogger::fatal(char const*, ...) const () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#7 0x00007fd12356af54 in QtWaylandClient::QWaylandDisplay::checkError() const () from /lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#8 0x00007fd12356bf9a in QtWaylandClient::QWaylandDisplay::flushRequests() () from /lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#9 0x00007fd125912b44 in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#10 0x00007fd1259345cc in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#11 0x00007fd1258d9b8b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#12 0x00007fd1258e2024 in QCoreApplication::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#13 0x000055cb21ad3e5e in ?? ()
#14 0x00007fd125245565 in __libc_start_main (main=0x55cb21ad2f10, argc=1, argv=0x7fff5f8f5978, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fff5f8f5968) at ../csu/libc-start.c:332
#15 0x000055cb21ad3f8e in ?? ()
[Inferior 1 (process 1127) detached]