frontends(xmlui): better handling of missing _xmluiAppend

frontends/src/tools/xmlui.py

@@ -386,12 +386,14 @@ class XMLUIPanel(XMLUIBase):
                     cont = self.widget_factory.createVerticalContainer(_xmlui_parent)
                     self._parseChilds(cont, node, ('widget', 'container'))
                 try:
-                    _xmlui_parent._xmluiAppend(cont)
+                    xmluiAppend = _xmlui_parent._xmluiAppend
                 except (AttributeError, TypeError): # XXX: TypeError is here because pyjamas raise a TypeError instead of an AttributeError
                     if _xmlui_parent is self:
                         self.main_cont = cont
                     else:
                         raise Exception(_("Internal Error, container has not _xmluiAppend method"))
+                else:
+                    xmluiAppend(cont)
 
             elif node.nodeName == 'tab':
                 name = node.getAttribute('name')