What exactly are you trying to do?
I have been using Qt Creator to view and read source code from various projects - I have no crashes - thus the question - what are you trying to do?
And my version of Qt Creator is 12.0.2-3
Can you produce a project which when loaded will cause the crash ?
Can you produce a set of steps which leads to a crash ?
I didn’t know such thing existed …
If you choose in Qt Creator → File → New → Qt template → Qt Designer Form - I reckon it will be something similar
It is quite possible the Design Studio is a commercial offer - thus there is no PKGBUILD because the source cannot be retrieved.
To get Qt Design Studio you need an Qt Account and Qt Account Login is the place to sign up.
Then you can use the Qt online installer to install into your home folder.