SUMMARY
I stumbled upon this when I was modifying a disk's partitions. When you create a new partition, assign it a mount point. But then don't actually click 'apply', next time you try to change that mount point, it crashes

STEPS TO REPRODUCE
(disclaimer: it might be that these steps don't need to be as exact, I'm just describing my exact scenario)
1. Take a disk with an existing partition.
2. Shrink that partition. (ntfs in my case)
3. Create a new partition in that freed up space (xfs in my case)
4. Right click the new partition and set a mount point (/Steam850EVO2TB for me, and enable 'users can mount or unmount')
5. (might have exited and reopened kde part manager here, not sure)
6. right click that partition again and remove the mount point using the 'remove' button.

OBSERVED RESULT
Terminal output: 

EXPECTED RESULT
Couldn't figure out how to verbose kde part manager, all it says in terminal is
zsh: segmentation fault (core dumped)  partitionmanager
The program just crashes the moment I click that delete button in the mount options

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
Kernel Version: 5.10.15-1-MANJARO
OS Type: 64-bit
Processors: 16 × AMD Ryzen 7 3700X 8-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: GeForce GTX 1080/PCIe/SSE2

Full: https://gist.github.com/thibaultmol/b845e46387db60129b02df77c08a36e5 

ADDITIONAL INFORMATION