DataSourceRelation field is empty in Part Properties

Hi

I added a FormPart to a form and I assigned to it a Datasource via the properties window.

But when I tried to assigned a DataSourceRelation, the list is empty, not even the SelfLink option:

Because of this (I think), the table is empty in the Part initialisation and it displays nothing.

What is populating the DataSourceRelation list ?

Why sometimes it is empty and sometimes it is not ?