Documents and Media - When editing a Document Type, the diagram builder canvas is blank when the edit window comes up

This doesn't happen in ee-6.1.x. In ee-6.1.x, the original fields are present when editing a document type.

Steps to reproduce:

1. Go to Control Panel > Documents and Media
2. Click Manage > Document Types and add a Document Type with a Boolean field
3. Click on the actions menu next to the created document type and click Edit
   • The boolean field won't be there, and there will be a Metadata field at the bottom that has the same name as the document type

Expected result:
The boolean field should be present in the build canvas. All the original fields ought to be there as this makes it easy for the user to edit the existing document type. There also shouldn't be any metadata fields because the user didn't add one when creating the document type.

Actual result:
The build canvas is empty, and there is a metadata field that has the same name as the document type.