How do I set field mapping in the Feeds module for Drupal 8? I'm trying to use Feeds 8.x-3.x-dev on Drupal 8.4. I've created a feed type (eg. News Feed) and I'm trying to set the field mappings, but I get an empty list with no way to add anything to it:

The target content type is a custom one, that's similar to Articles, but it also contains some extra fields (such as the source of the feed).
If I add a feed URL to this type and try to import it, it obviously fails:

Test Source has been created.
Warning message
An entity of type &quot;content&quot; failed to validate with the following errors:
Title (title): This value should not be null.
News GUID (field_news_guid): This value should not be null.
News Link (field_news_link): This value should not be null.
News Source (field_news_source): This value should not be null.
Please check your mappings.
Error message
Failed importing 20 News.

Most tutorials and references online talk about the module for Drupal 7, which looks significantly different from what I see.
Is there something I'm missing or is this mapping feature currently broken?
Edit: I am aware of the pre-release status of this module, but I'm trying to evaluate it to see what works and what doesn't. Downgrading to Drupal 7 isn't an option, so I have to make do with what I have for 8. If not Feeds, is there any alternative for a simple RSS/Atom feed import? Aggregator would work just fine for my needs, but how do I get it to import the contents as nodes?
Edit: I decided to downgrade to Drupal 7.
First, select a target to map to.

This will result into a list of sources to appear. Select one of these sources.



Notes:


The UI for CSV parser is still incomplete. See https://www.drupal.org/node/2669974
If the sources list does not appear after selecting a target, you may be facing javascript issues. In this case, check with a debugger which scripts fails. In D7, the jQuery Update module (or how that modules was configured) sometimes was the culprit.
If there are no targets appearing, try to rebuild caches.
Currently there is a bug with updating items if the target 'Feeds item' is not in the list. Be sure to map to that target if you not only want to insert new items, but also update existing ones. See https://www.drupal.org/node/2857692.