Xcode simulator

I've recently downloaded Xcode 8.and am having problems with simulator. I'm using Iphone 7 plus. t compiles okay but when running gives me the error message "Failed to initiate service connection to simulator" and "unable to contact local DTServiceHub to bless simulator connection." I had the same problems with Xcode 8.3.2. I have a 2009 MacBook with Sierra recently installed. I've tried restarting computer, rebooting simulator, resetting contents of simulator, recreating simulator, and nothing has worked.