Make 'default_version` field as readonly if no active versions are found.

[dojutsu-user]

If there are no active versions of a project, an error is raised when Advanced Settings page is accessed.

[stsewd]

Looks like always expect a value there

https://github.com/rtfd/readthedocs.org/blob/4737c835c9ba039c39a83204179060e329446c21/readthedocs/projects/models.py#L146-L151

What happens if the user tries to save the form?

[dojutsu-user]

@stsewd
The value is set to latest only. Only the field is made readonly.

After saving the form, build is triggered and successful but http://localhost:8000/docs/requests-test/en/latest/ results to 404

[dojutsu-user]

https://github.com/rtfd/readthedocs.org/blob/4737c835c9ba039c39a83204179060e329446c21/readthedocs/projects/forms.py#L243-L246

These lines were causing the problem if the active_versions results to [()] from :
https://github.com/rtfd/readthedocs.org/blob/4737c835c9ba039c39a83204179060e329446c21/readthedocs/projects/forms.py#L272

[ericholscher]

Seeing this in prod: https://sentry.io/organizations/read-the-docs/issues/857418442/?project=148442&query=is%3Aunresolved&statsPeriod=14d&utc=false

[dojutsu-user]

@ericholscher
As sentry logs are hidden from me .. can you please update me as what needs to be done here.

[ericholscher]

Mostly I was just saying it's a priority because users are hitting it, which is a really bad UX :)

[dojutsu-user]

This was introduced by me in #5157.
I am sorry for it.