[expander] do not pass NULL to the foreach callback
[expander] do not pass NULL to the foreach callback

Check for NULL before passing the child to the callback.

Fixes http://bugzilla.moblin.org/show_bug.cgi?id=9917

jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx