Prevent pasteTarget selection while already selected

onModelSelect is used to restore focusable node selections when a
dialog is closed, but the logical refactoring to do this was overly
generous in that this.focusedNode was allowed to clear, and the
pasteTarget was re-focused, emitting extra unhelpful events.

Change-Id: I466a64364fa5c38f177354b28ebfad02fb9df1a0