The error should be fixed now in build 8861.
It was caused by not finding any iOS Simulator version on your system, so maybe something has changed about the location of the simulator and the detection of it has to be updated as well.
2015-12-15 10:34:27,297 DEBUG [AWT-EventQueue-0] stencyl.sw.util.FileHelper: Failed to list contents of /Applications/Xcode.app/Contents/Developer/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs. It does not exist or is not a directory.
2015-12-15 10:34:27,297 DEBUG [AWT-EventQueue-0] stencyl.sw.util.FileHelper: Failed to list contents of /Applications/Xcode.app/Contents/Developer/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs. It does not exist or is not a directory.
2015-12-15 10:34:27,297 DEBUG [AWT-EventQueue-0] stencyl.sw.editors.game.mobile.MobilePage: Found the following simulator versions: []
2015-12-15 10:34:27,336 ERROR [AWT-EventQueue-0] root: Unexpected problem on thread AWT-EventQueue-0: null