Invalid command line
well, then that’s exactly the case where you should fix UVS.
The only way i’m aware of is through the launcher, by clicking “Fix associations” when popup arise. The popup should look like this:
After than *.project files should be able to be properly started by UVS.
The tricky part is: i don’t know how to reliably trigger this popup other than by installing new engine from launcher (or, probably, by starting launcher while having other launcher engines installed). That’s troublesome when the only engine you need is an a custom installed version.