Fixed various NULL reference issues. Also cleaned up wxPGComboBox code overall a bit.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56782 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/propgridiface.h

@@ -363,7 +363,10 @@ public:
     */
     const wxPGAttributeStorage& GetPropertyAttributes( wxPGPropArg id ) const
     {
-        wxPG_PROP_ARG_CALL_PROLOG_RETVAL(*((const wxPGAttributeStorage*)NULL));
+        // If 'id' refers to invalid property, then we will return dummy
+        // attributes (ie. root property's attributes, which contents should
+        // should always be empty and of no consequence).
+        wxPG_PROP_ARG_CALL_PROLOG_RETVAL(m_pState->DoGetRoot()->GetAttributes());
         return p->GetAttributes();
     }