Xcode Simulator in use

Cannot launch in simulator while it was already in use. IPhone Simulator appeared when running another program, what is wrong with this error?

Your simulator is in use. In this case, it is generally your other project that is running the occupancy simulator, and of course it may be that the system is not released when you end up running.
Solution:
(1) Close other works, leaving only the current project to run, then run,
(2) exit the simulator (COMMAND+Q), then run the project you want to run,
(3) exit Xcode, reopen, run your project, and
(4) Restart Mac OS X. The
above plan is progressive, one can't take the next, General (1) (2) is enough