Initialized the controls on the widgets spinbutton page

Initialized the controls on the widgets spinbutton page
and checked for NULL in one of the event handlers, since
on the Mac the event handler may be called before the
control is properly initialized
Tested for an empty string in Mac's textctrl.cpp
else we get a crash if there is nothing in the text control

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15555 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775