It sounds like you're using CuraEngine as the slice engine, and there seems to be some flawed logic with the error message that's appearing for infill.
I'll make a bug report for it so the dev team can address it; in the meantime I suggest either switching to MatterSlice so you can use LINES as the infill type or using a lower infill density with CuraEngine.
MatterControl doesn't really work with the 5th generation Makerbots. You can export X3G files for them to print from the SD card, but you cannot connect to them directly over USB. The 5th gens use a different communications protocol than most printers, which we don't support (yet).
Nevertheless, the application should not be crashing under any circumstances. Could you run MatterControl from the terminal and post the output?
Sorry. We tried to replicate your issue using our Flashforge Creator Pro, but we were not able to do it. Maybe there is some difference between the Flashforge and the CTC that we are not aware of. Without having one here for testing, there is not much we can do.