ConsistencyMessage: Make Constraint Usage facultative

product/ERP5Type/ConsistencyMessage.py

@@ -45,6 +45,10 @@ class ConsistencyMessage(ObjectMessage):
     init specific variable to constraint
     """
     ObjectMessage.__init__(self, object_relative_url, message, mapping)
+    self.description = ""
+    self.class_name = "Constraint Message"
+    self.constraint_relative_url = ""
+    if constraint:
       self.description = constraint.description
       self.class_name = constraint.__class__.__name__
       # keep track of the relative URL of the constraint to have it included in