And there is a crash after closing the window:

#0  0x00007f0f89cafd22 in raise () from /usr/lib/libc.so.6
#1  0x00007f0f8b6c8540 in KCrash::defaultCrashHandler(int) () from /usr/lib/libKF5Crash.so.5
#2  <signal handler called>
#3  0x00007f0f89cafd22 in raise () from /usr/lib/libc.so.6
#4  0x00007f0f89c99862 in abort () from /usr/lib/libc.so.6
#5  0x00007f0f8a0e5910 in QMessageLogger::fatal(char const*, ...) const () from /usr/lib/libQt5Core.so.5
#6  0x00007f0f8a0e61ae in ?? () from /usr/lib/libQt5Core.so.5
#7  0x00007f0f8ba00df8 in DeviceScanner::~DeviceScanner (this=0x558de2f96ad0, __in_chrg=<optimized out>) at /usr/src/debug/build/src/kpmcore_autogen/TAC5DWH4SE/../../../../kpmcore-21.12.0/src/core/devicescanner.h:25
#8  DeviceScanner::~DeviceScanner (this=0x558de2f96ad0, __in_chrg=<optimized out>) at /usr/src/debug/build/src/kpmcore_autogen/TAC5DWH4SE/../../../../kpmcore-21.12.0/src/core/devicescanner.h:25
#9  0x00007f0f8a338bee in QObjectPrivate::deleteChildren() () from /usr/lib/libQt5Core.so.5
#10 0x00007f0f8ae225ce in QWidget::~QWidget() () from /usr/lib/libQt5Widgets.so.5
#11 0x0000558de26a0be4 in MainWindow::~MainWindow (this=0x558de2eedd80, __in_chrg=<optimized out>, __vtt_parm=<optimized out>)
    at /usr/src/debug/build/src/partitionmanager_autogen/DMHXEJ42XS/../../../../partitionmanager-21.12.0/src/gui/mainwindow.h:39
#12 MainWindow::~MainWindow (this=0x558de2eedd80, __in_chrg=<optimized out>, __vtt_parm=<optimized out>) at /usr/src/debug/build/src/partitionmanager_autogen/DMHXEJ42XS/../../../../partitionmanager-21.12.0/src/gui/mainwindow.h:39
#13 0x00007f0f8a33b310 in QObject::event(QEvent*) () from /usr/lib/libQt5Core.so.5
#14 0x00007f0f8b97aaf8 in KXmlGuiWindow::event(QEvent*) () from /usr/lib/libKF5XmlGui.so.5
#15 0x00007f0f8ade3d62 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
#16 0x00007f0f8a30e41a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /usr/lib/libQt5Core.so.5
#17 0x00007f0f8a311519 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/libQt5Core.so.5
#18 0x00007f0f8a367a14 in ?? () from /usr/lib/libQt5Core.so.5
#19 0x00007f0f8898d52c in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#20 0x00007f0f889e17b9 in ?? () from /usr/lib/libglib-2.0.so.0
#21 0x00007f0f8898ac11 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#22 0x00007f0f8a367046 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#23 0x00007f0f8a30cd8c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQt5Core.so.5
#24 0x00007f0f8a3152f4 in QCoreApplication::exec() () from /usr/lib/libQt5Core.so.5
#25 0x0000558de269f826 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/partitionmanager-21.12.0/src/main.cpp:77

Can you paste output of

lsblk --nodeps --paths --sort name --json --output type,name

{
   "blockdevices": [
      {
         "type": "disk",
         "name": "/dev/nvme0n1"
      },{
         "type": "disk",
         "name": "/dev/sda"
      }
   ]
}

(In reply to Matej Mrenica from comment #3)
> {
>    "blockdevices": [
>       {
>          "type": "disk",
>          "name": "/dev/nvme0n1"
>       },{
>          "type": "disk",
>          "name": "/dev/sda"
>       }
>    ]
> }

Hmm, that looks fine, no "/tmp/dev/sda" anywhere here.

Does partitionmanager print

Loaded backend plugin:  "pmsfdiskbackendplugin"

when you start it from terminal. I suspect you might be trying to use dummy backend instead of sfdisk backend.

Dummy backend is unfortunately broken right now (but in any case it's not useful for users as it doesn't do anything).

(In reply to Andrius Štikonas from comment #5)
> Does partitionmanager print
> 
> Loaded backend plugin:  "pmsfdiskbackendplugin"
> 
> when you start it from terminal. I suspect you might be trying to use dummy
> backend instead of sfdisk backend.
> 
> Dummy backend is unfortunately broken right now (but in any case it's not
> useful for users as it doesn't do anything).

No, it shows:
Loaded backend plugin:  "pmdummybackendplugin"
>  you might be trying to use dummy backend instead of sfdisk backend.
I am not telling it to do so tho.

(In reply to Matej Mrenica from comment #6)
> (In reply to Andrius Štikonas from comment #5)
> > Does partitionmanager print
> > 
> > Loaded backend plugin:  "pmsfdiskbackendplugin"
> > 
> > when you start it from terminal. I suspect you might be trying to use dummy
> > backend instead of sfdisk backend.
> > 
> > Dummy backend is unfortunately broken right now (but in any case it's not
> > useful for users as it doesn't do anything).
> 
> No, it shows:
> Loaded backend plugin:  "pmdummybackendplugin"
> >  you might be trying to use dummy backend instead of sfdisk backend.
> I am not telling it to do so tho.

Is it in the settings? ~/.config/partitionmanagerrc

If yes, then either adjust it there or remove config file. If not, then somehow real sfdisk backend is not installed or broken.

*** This bug has been marked as a duplicate of bug 432704 ***

I deleted the config file and now it works.