This bug was introduced after setting dockable property
inside 'Objects list' (version 1.41b70).
1. Goto a first tab.
2. Select an object, but not the first.
3. Goto a second tab.
4. Select another object (also not the first).
5. Switch between them.
6. You'll see an 'empty' tab (it's not connected with
current docking or not).
Problem is probably in different (than before docking)