Error when saving an overridden Document Type

Step to reproduce the issue:

• Add a project as dependency in the `Application Definition`
• In the Dependencies menu, browse the `Document Types`, select one and click on the button `Override`
• In the Document Type editor, click on the `Schema` tab
• Add a new field in the fields table
• Click on `Save`

=> A validation error is displayed

Validation error: The prefix 'mydoc' already exists in the default application.