core (xmlui): fixed data_form => XMLUI conversion when filters is None

src/tools/xml_tools.py

@@ -118,6 +118,8 @@ def dataForm2Widgets(form_ui, form, read_only=False, prepend=None, filters=None)
         This is especially useful to modify well known fields
     @return: the completed XMLUI instance
     """
+    if filters is None:
+        filters = {}
     if form.instructions:
         form_ui.addText('\n'.join(form.instructions), 'instructions')