[Transfer] Error if 'destination' is NOT specified

Following output if destination is NOT specified:

[INFO] OK
[INFO] Transferring files encrypted for christian.ribeaud@karakun.com
[ERROR] FAILED
upload() missing 1 required positional argument: 'destination_dir'

I think, the best way to solve this is to disable buttons Test and Transfer selected files as long as mandatory fields are empty.

And make sure that the program actually reads the value. It might be a Mac specific issue but, if I do NOT leave the field (field still having the focus) to confirm the input, field is still considered empty.

The deluxe implementation would be to deactivate buttons as soon as mandatory fields are NOT filled in.