Steps to reproduce:
1. Upload one or two tracks and select trim in the workflow
2. In select tracks unclick all the tracks
3. Press "Continue Processing"
Workflow resumes and then fails as there are no tracks selected
According to the QA, you should not be able to unselect all the tracks or at least not be able to proceed unless at least one is selected.
I'm not sure if is relates to as it is inconclusive as to whether anything was changed.