!selection.show => return null from getSelection
!selection.show => return null from getSelection

If we've called `clearSelection`, then calling `getSelection` shouldn't
return the old selection values. When calling `getSelection` before a
selection is made, `null` is returned. It seemed appropriate to do the
same if `selection.show` was falsy.

Signed-off-by: Nick Campbell <e31591b49bf5c9127aafb098f2b9d466891b32f9@gmail.com>

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