A failure happens.
Some internal state variable are not initialized.
This makes the get_snapshot() method explodes.
Failing completely to update the visit.
Adding the proper initialization in the __init__ method should fix it.
(That will associate an empty snapshot to the failed visit).
This should fix issues like [1]
https://sentry.softwareheritage.org/share/issue/ace9143c0f6244b49b18cca49a5aa809/