Just installed the latest version of flightgear from the EDIT: Flightgear PPA stable repository on Linux Mint. The launcher displays an empty window while fgfs itself runs fine.
Turns out that it needs qtquick2, this would be a quick patch just to add it as a dependency. I'm sure lots of people are installing from the repository to find that it does nothing!
qml-module-qtquick2 is not enough on its own, I suspect one of the controls or window packages is also required, but to save myself the effort I just pulled in qml-module-qtquick-* to get it running.