When I add an Aggregate, the first entity must be dragged and dropped to the widget from the entities panel.
But, after adding one entity the widget shows a "Add Source" link in the sources tab.
For me, the Add Source is the fastest and more productive way to add a source. It's the same process we were used to with the simple queries.
My request is only to have the 2 possibilities with empty Agrregates: drag & drop OR add source