elm_widget: do not set NULL as parent for object

When widget has no parent object, it breakes usage of a custom
Elm_Theme. This commit fixes it.

@fix

legacy/elementary/src/lib/elm_widget.c

@@ -1289,7 +1289,7 @@ _elm_widget_sub_object_del(Eo *obj, Elm_Widget_Smart_Data *sd, Evas_Object *sobj
           }
 
         ELM_WIDGET_DATA_GET(sobj, sdc);
-        sdc->parent_obj = NULL;
+        sdc->parent_obj = elm_widget_top_get(obj);
      }
 
    if (sd->resize_obj == sobj) sd->resize_obj = NULL;