ERP5Workflow: give a more explicit error when reference is not set

it was 'TypeError: cannot concatenate str and NoneType objects'

product/ERP5Type/id_as_reference.py

@@ -73,6 +73,8 @@ def IdAsReferenceMixin(extra_string, string_type="suffix"):
     def _setReference(self, value):
       parent = self.getParentValue()
       self.__dict__.pop('default_reference', None)
+      if value is None:
+        raise ValueError('Reference is not set.')
       if string_type == "prefix":
         new_id = extra_string + value
       elif string_type == "suffix":