By design, the EMF Validation framework runs checks from a read-only environment. The model cannot be modified from a check.
If you want a fix, you should add it to the "fix" entry in your constraint. I'm not sure what your constraint is trying to do though: wouldn't it keep adding fields whenever you had zero, two or more fields?
Please explain what you are trying to check for. Do you want to make sure that the model object has an ownedField with a text different from "Text"?