The problem: While creating a workflow process in Dynamics 365 (CRM Online), the "Form Assistant" drop-down boxes (i.e., where you can select the "Operator" or dynamic values) were empty/blank.
What we tried: We tried these steps to troubleshoot the issue: 1) click into a variety of fields to see if the problem is related to specific types of fields (no, the options are blank for all field types), 2) clear browser cache, 3) try IE, Chrome and Edge, 4) determine whether the problem is happening for all entities.
Some clues: Regarding #4 above, the problem with the blank options when editing a workflow step occurred for some entity types but not all. So what was common between the entity forms where this bug occurred?
The fix: It turns out, at least for the CRM organization we're working with, that if there's an editable grid on the form, then the Form Assistant controls do not get populated. We removed the editable grid, saved, published and then tried editing the workflow step again. That resolve it: The expected operators, field choices, etc. are appearing in the workflow step edit page again.
This might be a coincidence, but this work-around is worth trying if you run into the same problem.