Validate constraint doesn't seem to activate

1. Create a formalism with a class.
2. Add a string attrib to the class.
3. Place an action on the class with code that checks whether the attrib is 'abc'
4. Create concrete syntax and instance model
5. Change attrib in instance model to '123'
6. Click verify button and observe that no error is raised.

Reproduce the issue by creating a formalism with a class, string attribute, and action constraint, then create the concrete syntax and instance model. Start at the class action and the Verify button flow; done means changing the attribute to "123" and verifying raises an error for the constraint requiring "abc".

Written by the indexing model from the issue text.