[Bug] beta.pybricks.com Press "Backup all files" with no files is reported as "An unexpected error"

[BertLindeman]

Describe the bug
Pressing "Backup all files" while there are no files is reported as:

An unexpected error occurred. Please consider reporting this so we can make a better error message.

To reproduce
Steps to reproduce the behavior:

1. Go to https://beta.pybricks.com/
2. Make sure there are NO files listed in the file explorer
3. Press the button "Backup all files"
4. See error

An unexpected error occurred. Please consider reporting this so we can make a better error message.

Expected behavior
An error like "There are no files to backup"

Screenshots
None now

Details:
Version v3.2.0b1

Error: no files
    at https://beta.pybricks.com/static/js/main.bc353f8e.js:2:5261332
    at d (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651544)
    at Generator._invoke (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651297)
    at Generator.next (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651907)
    at d (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651544)
    at C (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4652667)
    at Generator._invoke (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651039)
    at Generator.next (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4651907)
    at d (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4895696)
    at u (https://beta.pybricks.com/static/js/main.bc353f8e.js:2:4896029)