When user clicks collapsed timeline group, all entities inside group are also selected.
`UserInterface.activeSelections` returns following items:
* one `Selection` item for each entity inside group with corresponding 'entity' property
* one additional `Selection` item with `entity` set to null
It seems this additional item represents group itself, but why 'entity' is not set?
Moreover, if you select timeline group and then try to do
```
sel = ui.activeSelections.all
ui.activeSelections.all = sel
```
Then `RuntimeError: 3 : invalid argument entity` is returned.
Same error is returned if you try to explicitly select timeline group using
```
ui.activeSelections.add(timelineGroup)
```
