As the GDB said:
This generally means that another instance of this process was already running or is hung in the debugger
So find out it and kill it. ;)
I restarted the mac, and it run successfully.
Tip: Well, this may be not work sometimes, so I strongly suggest that you press cmd+. to make sure that you do have stopped the simulator (or device). I find that in most cases the simulator (or device) will not stop exactly when it crashes. I rarely met this issue again with pressing the cmd+. every time it crashes or even just normal exit. Hope this will be helpful. :)