Krita tends to crash when copy-pasting something, if not the first time then likely the second time.
This is a bug (https://bugs.kde.org/show_bug.cgi?id=502266) related to the clipboard system, and it has been fixed in 5.2.10. This isn't available in the Artix repo yet, so I solved this by manually downloading the Arch package and installed that using pacman -U.