Might have found the answer.
Crash on Catalina not Mojave so guessing it was 64bit code thing.
I had recently update my Audio interface from MR816 to AR4X but had not uninstalled the related tools for MR which I believe are installed into the OS kernel. Ran the uninstaller on Catalina from the last MR tools set for a) MR Editor b)MR Extensions c) FW driver. Cubase is likely use these code elements so becomes unstable.
Crash no longer occurs. Heres hoping I guessed right.