Error: 'cannot marshal None unless allow_none is enabled' while executing 'ticket.version.get()'

[Steffen Pingel]

After setting up a fresh Trac repository and trying to fetch version information through the XmlRpcPlugin Trac displayed an error:

'cannot marshal None unless allow_none is enabled'

when executing 'ticket.version.get()'.

This is caused by the time field of the version which is set to NULL in the database.