SUMMARY
This has been seen on the latest 4.3.0-prealpha (git b3cde41) appimage.

STEPS TO REPRODUCE
1. With restarted krita and a new image, paint some 'blobs' for use as selected objects.
2. Use the magnetic Selection Tool to create a selection around an object.
3. Deselect the resulting selection and make a fresh selection using the Magnetic Selection Tool.
4. Note that this 'Deselect - make new selection' can be repeated.
5. With a made selection, use any tool such as Freehand Brush or Move Tool - note that this works.
6. Try to use the Magnetic Selection Tool again and note that it does not place nodes.
7. Note that other tools still work.
8. Close the image and make a new one.
9. Repeat the above and when the Magnetic Selectiom Tool is not working, try repeated clicking and right-clicking. There will be a lock-freeze with 100% CPU usage for about 15 seconds followed by a crash.

OBSERVED RESULT
See above. See Additional Notes

EXPECTED RESULT
It should keep working if used and should not lock/crash

SOFTWARE/OS VERSIONS
Krita

 Version: 4.3.0-prealpha (git b3cde41)
 Languages: en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.5
  Version (loaded): 5.12.5

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-6-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10

Hardware Information

  GPU Acceleration: none
  Memory: 16039 Mb
  Number of Cores: 8
  Swap Location: /tmp


ADDITIONAL INFORMATION
When a selection had been made, the 'move selection' cursor will appear over the selection boundary but it will not move the selection outline. Sometimes it will place a node that can be removed by pressing Esc. Sometimes it will result in a failure of the Magnetic Selection Tool to work any more, as in Step 6. above.