Entity class name is not correctly prefilled when deselecting classes in Entity tree

[soininen]

1. Start DB editor with empty database.
2. Add two new entity classes, A and B
3. Select A, then B. Observe that the entity_class_name cell in the parameter and entity alternative tables is prefilled with B because it was selected last.
4. Deselect A.
5. Note that entity_class_name on the empty row is now A. This is incorrect; it should be B since it is the only class that is currently selected.