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.