null values: If your Jira expression returns nullit is considered a non-boolean value and hence Jira returns an error. For example, to check that the issue is assigned, if you provide the Jira expression as:
when tested against an issue that is unassigned the expression returns null. In such cases, the easiest is to use two logical not operators (!!) to return true if the issue is assigned ( !issue.assignee returns false hence !! issue.assignee will return true when the issue is assigned).
You can also write a Jira expression to check that the result is not null like this:
If your Jira expression accesses properties of an object that is null, then your Jira expression and thereby your workflow condition/validator fail with an error. For example, to check that the issue's parent is a Story if you provide the Jira expression:
issue.parent.issuetype.name == "Story"
when tested against an issue without a parent, the Jira expression will return an error "Type null does not have any properties". To handle this you should include an expression to test that the issue has a parent.
!! issue.parent && issue.parent.issuetype.name == "Story"