How can I add a workflow transition condition to make sure a role has a user assigned, which add on or plugin is required for this?
I need to make Assignee field mandatory but in condition I am not having any option to make the user mandatory. So please tell how to get such option in this.
Those both sound more like validators than conditions, but you can do them both.
Off the shelf, "make sure a role has a user assigned" is easy - you can just use "user is in role x". That guarantees that role X has at least one user in it.
Making assignee mandatory is definitely better done as a validator (and there's one in the JSU add-on, if not the core, for requiring system fields). As a condition, I think you'll need to find/write something, as most people do it as a validator.
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