rake failing when trying to open ios simulator

I'm following the HelloRuby tutorial. I recently downgrated from El Capitan to Yosemite on my Mac Pro for many reasons. At any rate, my Xcode version is: Version 7.2.1 (7C1002) which should be right for Yosemite. Here's what happens. I get what you see below. This example works fine on my laptop (running El Capitan). Any thoughts? I'm thinking I should completely uninstall Xcode and reinstall from scratch, but otherwise I'm at a loss.

HelloMotion$ rake
Build ./build/iPhoneSimulator-9.2-Development
Create ./build/iPhoneSimulator-9.2-Development/HelloMotion.app/Info.plist
Simulate /Users/Stan/Dropbox/git/HelloMotion/build/iPhoneSimulator-9.2-Development/HelloMotion.app
2016-10-10 09:18:25.495 repl[621:6201] +[SimServiceContext sharedServiceContextForDeveloperDir:error:]: unrecognized selector sent to class 0x10afc68c8
2016-10-10 09:18:25.503 repl[621:6201] **\* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[SimServiceContext sharedServiceContextForDeveloperDir:error:]: unrecognized selector sent to class 0x10afc68c8'
**\* First throw call stack:
(
0 CoreFoundation 0x00007fff95b3d03c **exceptionPreprocess + 172
1 libobjc.A.dylib 0x00007fff941b676e objc_exception_throw + 43
2 CoreFoundation 0x00007fff95b3ffad +[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x00007fff95a85e24 ___forwarding_** + 1028
4 CoreFoundation 0x00007fff95a85998 _CF_forwarding_prep_0 + 120
5 repl 0x00000001093931f2 +[RMSimDevice defaultDeviceSet:] + 66
6 repl 0x00000001093a0c17 _Z17init_IOSSimulator19jit_bridge_config_t + 775
7 repl 0x00000001092f0609 main + 5289
8 libdyld.dylib 0x00007fff9b4a85c9 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
#

The application terminated. A crash report file may have been generated by the
system, use `rake crashlog' to open it. Use`rake debug=1' to restart the app
# in the debugger.

rake aborted!
TypeError: no implicit conversion from nil to integer
/Library/RubyMotion/lib/motion/project/template/ios.rb:243:in `exit'
/Library/RubyMotion/lib/motion/project/template/ios.rb:243:in`block in '
Tasks: TOP => default => simulator
(See full trace by running task with --trace)
HelloMotion$

Start by reproducing the failure with rake in the HelloMotion project on the reported Yosemite and Xcode 7.2.1 setup, then inspect the simulator launch path and the stack trace references to /Library/RubyMotion/lib/motion/project/template/ios.rb:243. Done means rake launches the iPhone simulator without the SimServiceContext crash or the nil-to-integer error.

Written by the indexing model from the issue text.