After adding new entity fatal when creating relation

Problem/Motivation

This issue is based on #2315599: Consider Dynamic Entity Reference for storage needs patch #59.
When a relation type is created and a new entity is added we get a fatal on relation/add page
because the selection handler is not defined.

Steps to reproduce:
1. Uninstall taxonomy module
2. Create a relation type
3. Enable taxonomy module
4. go to relation add page: relation/add

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

This issue based on #2315599: Consider Dynamic Entity Reference for storage needs