The current Feedback app has problems on a number of the latest Linux distros including Mint 19, SuSE Enterprise Desktop 15, and Fedora 29. For me, I get the same libmodmap error followed by a core dump.
Since the libmodmap error was solved in 2018r2, I would expect a rebuilt version of Feedback using 2018r3 to inherit that fix.
Well, we got what I was asking for and it runs properly on Mint 19, Ubuntu 18.04/18.10, Fedora 29, and SuSE LEAP 15.1.
But, they didn’t pay attention to the pinned entry at the top of the Linux targets list and use the modGTK3 module to make things look clean across the board.