Cannot use relationship for rendered entity on Views

Problem/Motivation

You cannot create a view and try to list rendered entities using relationship.

Steps to reproduce (based on Standard profile):

1. Add an entity reference field named "field_content" to the "page" content type. Allow content > article references.
2. Create and edit a content view.
3. Add a relationship for "Content using field_content."
4. Change row style plugin to "Content" and hit "Apply".
5. An AJAX error occurs.

The AJAX error is caused by fatal error triggered when RowPluginBase::buildOptionsForm() calls RelationshipPluginBase::init() while providing a wrong argument type (Array instead of DisplayPluginBase).

Proposed resolution

Fix it

Remaining tasks

Fix it, by passing along the relationship as needed and using it.

User interface changes

API changes