Receiving “Type null does not have any properties“ error

Often, you want to check some sub-property of an element, for example, the display name of the assignee. In this case, you would write something like:

issue.assignee.displayName != "John Doe"

However, if the issue is currently unassigned, this will result in an error. You can fix this by checking that an assignee is set for the issue explicitly:

issue.assignee != null && issue.assignee.displayName != "John Doe"

Note that setting a field to hidden in a field configuration (i.e. disabling the field) will result in the corresponding field evaluating to null, which could also result in this error.