Fix Exception (no sequence executor) when press Play after Execution Stop/Start.
Essentially we just hook the Execution state changed event and free up our context on 'Closing' (it is going to be release anyway) and recreate it when Execution returns to 'Open' state.

This change also avoids the no executor Exception on Play after SetupDisplay/Cancel.  However, something is still wrong.  The sequence looks like it is playing in the editor, but the Preview doesn't show anything. Clearly something is not getting re-connected by I don't know what.  Closing/Opening the editor returns things to normal.

Maybe you guys can find what I've missed here?

Either way, the crash is gone in both cases, so I think this is worth checking in.