WARNING: Trying to access array offset on value of type null

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I am getting the following error message although the backup completed successfully:

[14-Jul-2021 13:18:18] Starting upload to S3 Service …
[14-Jul-2021 13:18:21] WARNING: Trying to access array offset on value of type null
[14-Jul-2021 13:18:21] Backup transferred to https://xyz-backup.s3.amazonaws.com/my_bucket/2021-07-14_13-17-31_CSVXIZCY01.zip.
[14-Jul-2021 13:18:21] WARNING: Trying to access array offset on value of type null

Any idea of what’s causing this error?
WP 5.7.2, PHP 8.0.8, BackWPup 3.9.0