Resolve ambiguity of calling overloaded wxPropertyGrid::SendEvent()

The right overloaded SendEvent() function is pointed out by explicitly
casting second argument of the call to (wxPGProperty*) type.

src/propgrid/manager.cpp

@@ -1797,7 +1797,7 @@ void wxPropertyGridManager::OnToolbarClick( wxCommandEvent &event )
         if ( DoSelectPage(index) )
         {
             // Event dispatching must be last.
-            m_pPropGrid->SendEvent( wxEVT_PG_PAGE_CHANGED, NULL );
+            m_pPropGrid->SendEvent( wxEVT_PG_PAGE_CHANGED, (wxPGProperty*)NULL );
         }
         else
         {