Removing Metafields results in undefined field in Discover

[J7mbo]

Kibana version: 7.6.0

Elasticsearch version: 7.6.0

Server OS version:

Browser version: Latest Safari

Browser OS version: Latest MacOS

Original install method (e.g. download page, yum, from source, etc.): Docker

Describe the bug: Removing Metafields in advanced, results in an undefined field in the browser that cannot be removed.

Steps to reproduce:

1. Go to advanced
2. Remove all meta fields (_source etc) and save
3. Go to discover and see that there is an empty field
4. Re-add either one of the meta fields or a random string in advanced again
5. Go back to discover, see that field is there
6. Understand it is impossible to remove all metafields

Expected behavior: There are no meta fields and no 'undefined' field.

Screenshots (if relevant):

Metafields Removed

and result in Kibana, undefined field:

Now adding random string that I don't care about:

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[lizozom]

Tried and was able to reproduce the empty undefined field appearing when all metaFields are removed.
Another bug is that when changing metaFields in advanced settings, the list of fields in discover is not updated.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)