Facet search_filter flag creating error for nested fields

[nichgray]

There's a chance this may not be related to the API, but adding it here for now for safekeeping:

When search_filter flags are added to facets with nested fields, it creates a search results error.

Here is the facet config from letters.yml:

Here is the error when you try to get search results:

And here's the log message from the API:

[nichgray]

@wkdewey fixed this!