from the exceptions printed from your screen .. i could deduce 2 things.
you clicked the WPA button. then you clicked the browse button but did not select any wordlist..the "index error" was generated because the application could not process
the input ,because no wordlist was selected.if i used a message box to prompt user.That would be disturbing because each time the user clicked cancel the message box would execute again and again..the program uses "killall" command to kill processes. so the "aircrack-ng: no process killed" simply means that by the time you clicked the WPA button, aircrack was not killed because it was not found running in the memory.if the application is suddenly killed it only kills its created threads, but since the aircrack suite is an independent process being called..fern does not reach to kill it after a sudden close of the entire program. aircrack and its processes are controlled and killed at certain events.. You can browse the source code to find them (can't list all). but you could also kill all processes by the scan button.