That launcher is a relatively "new" addition to FlightGear, it's using Qt5/QQ - and whenever you build from git/from source, you should understand that things are not necessarily "stable" at all.
That being said, the launcher is entirely optional, too. You use any other launcher, or even no launcher at all, i.e. by invoking fgfs directly. For details, just run fgfs --help --verbose. Depending on your OS/build, you may have to set the FGROOT environment variable first for that to work.
For details, see:
http://wiki.flightgear.org/$FG_ROOThttp://wiki.flightgear.org/Command_line_optionsPS: The screen shot you posted is actually showing a different kind of error, due to missing privileges/permissions. But you still aren't even running fgfs here, just a script intended to invoke it.