This issue is slightly different from "An illegal choice has been detected" when changing views and displays → , because the error occurs even when I do not touch the previously filled viewsreference fields.
Inside a views reference field it is not possible to select a Display of the selected parent View anymore. The select list remains empty: "There is no display available. Please select another view or change the field settings."
This error only occurs if the "Views reference autocomplete" widget is set. With the "Views reference select list" widget it works as expected.
When I edit an existing node that contains view references and I click on "save", it results in "An illegal choice has been detected." being thrown out.
I built up my site under drupal 8.3.7 and moved to drupal 8.4 two weeks ago. The error seems to only occur under drupal 8.4, but both under viewsreference_8.x-2.0-alpha2 and viewsreference_8.x-2.0-alpha1.
Fixed
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The debugging steps in this issue where really helpful, thanks. I identified that I had this problem on both widgets and after a bit of debugging realized that in the fields settings you have to set the allowed display modes. I've opened a new issue https://www.drupal.org/project/viewsreference/issues/3357451 ✨ There is no display available. Please select another view or change the field settings. Needs review