Fix multiple suggestion directory behavior when empty and required

1. add a multiple directory suggestion to a form (in studio)
2. click New to create a document
3. leave all required fields empty
4. add some datas in the non-required field
5. click Create
6. this generates the expected error
7. click again Create
8. again an error but the suggest field is now Read-Only
9. looking at the inspector shows some errors (see koReadOnly.png)
10. this happens only with 8.3 version and works fine on a 7.10HF17 version (see okNoError.png)

Please find 7.10 and 8.3 studio projects for faster reproduction.