Describe the bug
I was trying to add a random delay of 0-59 minutes per backup using the pre-backup command option. To do so I used the following command: sleep $[ $RANDOM % 60 ]m, but unfortunately while pre-backup commands are running, the Vorta UI freezes. This probably won't be an issue for most scripts, but will be a problem in cases where scripts take a long time to compete.

To Reproduce
Steps to reproduce the behavior:

1. Open Vorta -> Schedule -> Shell Commands
2. Enter a long running command as a Pre-backup command, such as sleep 1m
3. The Vorta UI will stop responding while the command is running

Desktop (please complete the following information):

• OS: Ubuntu 18.10