Currently (as of NW.js 0.88 and 0.89), there seems to be an issue where the process may sporadically not exit at least when playtesting, which prevents further playtests until RPG Maker MV is restarted.
NW.js 0.80 is reported to not have this problem. Certain versions in-between may work fine too - I recommend using the most recent version that is unaffected by this issue.