Only get selection_name if it exist

  Idea: This should be improved to reuse proxyfied field method
        instead directly.

product/ERP5Form/ProxyField.py

@@ -762,6 +762,7 @@ class ProxyField(ZMIField):
       return proxy_field.get_value(id, **kw)
 
   def _cacheable_key(self):
+    if self.has_value('selection_name'):
       selection_name = self.get_value('selection_name')
       if selection_name is not None:
         selection_tool = self.getPortalObject().portal_selections