SchemaGenerator: 'FooViewSet' object has no attribute 'format_kwarg'

[decentral1se]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Using the new schema generation via DefaultRouter, hook a ViewSet into it.

Expected behavior

A schema is generated for the ViewSet.

Actual behavior

AttributeError: 'AddressViewSet' object has no attribute 'format_kwarg'

[decentral1se]

I managed to avoid #4265 and #4278 but then ran into this.