frontends (xmlui): fixed crash on advanced_list use:

the data variable was set to None but kept for other nodes, resulting in a crash in a later test where a dict was expected.

frontends/src/tools/xmlui.py

@@ -377,9 +377,9 @@ class XMLUIPanel(XMLUIBase):
         @param wanted: list of tag names that can be present in the childs to be SàT XMLUI compliant
         @param data(None, dict): additionnal data which are needed in some cases
         """
-        if data is None:
-            data = {}
         for node in current_node.childNodes:
+            if data is None:
+                data = {}
             if wanted and not node.nodeName in wanted:
                 raise InvalidXMLUI('Unexpected node: [%s]' % node.nodeName)