Database List in class definition does only support string fields in refered class

As outlined here:

https://forum.xwiki.org/t/database-lists-with-class-name-id-and-value-field-data-type/16296

the Database List property of a class using the  “XWiki Class Name”, “Id Field Name” and “Value Field Name” options does only work if the referred fields are of type "String".

Either this should be validated when referring to a field in the class definition or a meaningful error should be displayed when the data is computed.

Right now, the list does simply not populate if either of the fields refer to a non String type.