Bug fix for Mac OS X when using wxPython 2.9.2.4 whereby wx.SafeYield() is causing 'Bus errors'.
The wx.SafeYield(None, True) calls have been replaced by wx.GetApp().Yield(True) calls, which should
do the same thing but do not cause bus errors on this developmental version of wxPython. This fix
is important as the future Mac OS X relax app will use this version of wxPython for 64-bit and Cocoa
support.
