When pdf is not added to pdf task participant will encounter spinning start button

[TjerkNan]

Describe the bug
A donation step contains a manual or other instruction that shows a PDF but PDF has not been uploaded to workflow item.
Participant will press start on this task but start button will just spin. Adding PDF to workflow and refresh by participant resolves this.

To Reproduce
Steps to reproduce the behavior:

1. Create data donation workflow
2. Add step with PDF document but don't add actual PDF
3. Start participant session and press start on 'PDF' task

Expected behavior

• A message that asks to contact the Creator/Researcher and inform them that a PDF was not attached.
• Another option is a terse error message 'no document uploaded' or something like that.
• A potentially better solution would be to put a yellow warning border around the task item at hand until a PDF is uploaded
• A variant would be a warning at the top of the workflow builder that an item is not complete

Screenshots
Blue start button keeps spinning.