Created Unassigned: Designer database first :: removing Model.edmx.diagram file makes designer surface usuable and throws bad exception when trying to perform any action on the model [1184]

Scenario:
- Designer database first - import any model from existing database.
- Remove Model.edmx.diagram file.
- Try to add/remove entity from the model.

Exception we throw is:
"XmlModels involved in this transaction are not editable."

Rebuilding solution or adding a new code generation item does not fix the problem.
I think customers can hit that, if the want a non-standard template - before you add a new template, you must remove the old one, and the diagram file is very close to the tt files, and could accidentally be deleted.