Use selected NID from Entity Reference field to populate second Entity Reference field
My solution is to:  


Pre-populate and disable the first entity reference field with a node ID
Add an extra path component with the same nid and have the second field use an entity reference view with default: Path Component 4


So the URL looks like /node/add/hacky/123?field_solution=123.

While hacky, it has uses, especially if you want to filter fields that don't have a connection. In that case the URL could be /node/add/hacky/123/456?field_solution=789.