Clear if missing pid

Hey @spenceralger,

Thanks for the handy tasks.   I've been running into issues with the process I'm launching with this dying on it's own (i.e. unhandled exception thrown), and not being able to re-launch it.   Tracked this down to the stop code, which wouldn't clear the _pid when the process was closed, thus preventing it from being restarted. 

sprinkled in a few clearPid() calls, and everything is happy again. 

lmk if you have questions,
steve