Fix crash when hammering the animation scrubber with clicks.

I found a crash when playing with animations in the editor.

`lastNode` is assumed to always be a descendant of `CCNode`, which isn't the case (the OutlineView also contains `SequencerJoints`). This results in an unrecognised selector exception which crashes the app. This PR fixes that by explicitly checking class membership after getting the item back from the view.