Versions are stored in lists. Documentation on how to work with lists can be found here.
You can access the whole version by issue?.versions[0] and issue?.fixVersions[0] which returns a Version object, issue?.versions (and issue?.fixVersions) will return a list of objects.
Read
Page properties
id
1
Returned value
The release date of the first (fix)version. If no (fix)version is available, null is returned.