Mapping is blank; cannot map field on local form

Description

Steps to reproduce

1. Create a page with a ContentArea.
2. Click a Create a new block link.
3. Select a Form container --> "New Block: Form container" page opens.
3. Dnd the element to the Form Container.
4. Click the Create button.
5. Open the form's Mappings tab.
6. Select Connect to Datasource.
7. Open the added element's Extra field mapping tab.

Expected:
You can map the external system field.

Actual:
External system field mapping is blank, and you cannot map it.