Allow returning NULL windows from GetCompositeWindowParts().

wxCompositeWindow::GetCompositeWindowParts() becomes simpler to implement in
the derived classes with optionally shown elements if NULL windows are allowed
(and ignored) in the list returned by it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68910 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/compositewin.h

@@ -110,7 +110,11 @@ private:
         {
             wxWindow * const child = *i;
 
-            (child->*func)(arg);
+            // Allow NULL elements in the list, this makes the code of derived
+            // composite controls which may have optionally shown children
+            // simpler and it doesn't cost us much here.
+            if ( child )
+                (child->*func)(arg);
         }
     }