First make a report with a description of how to replicate it in this topic:
Krita 5.2 Release Candidate is out! Please help us in another round of testing!
It’s like a staging area for discussing and investigating bugs before making a report on the formal bug tracker.
Those are the tricky ones. Try to give a detailed and precise description of how you induced the lockup. i.e. instructions for how to replicate it.