core(XMLUI): empty options in ListWidget now just show a warning message, no more assert

src/tools/xml_tools.py

@@ -608,7 +608,8 @@ class ListWidget(InputWidget):
         if style is None:
             style = set()
         styles = set(style)
-        assert options
+        if not options:
+            warning(_('empty "options" list'))
         if not styles.issubset(['multi']):
             raise exceptions.DataError(_("invalid styles"))
         super(ListWidget, self).__init__(xmlui, name, parent)