Convert gcylc stopped state to str in case it is None.

@benfitzpatrick - I have a suite that stops with a value of None in your "stopped state", for reasons I don't understand.  This prevents gcylc starting up again - it continually spawns the new error dialogs until killed.

This small change fixes the symptom, which is probably sufficient for now as I can't see any other deleterious effects.  But I'll tar up the suite and email it to you in case it's obvious to you what the root cause is.

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