Guard setState with isMounted

Firing two actions close together, where one action changes state for a watched store, and the other action results in the removal of the watching component from the DOM, can produce "Error: Invariant Violation: replaceState(...): Can only update a mounted or mounting component." from React.  The setState call therefore needs to be guarded even though componentWillUnmount removes the listener.

I haven't written a test for this, yet.