make popups more predictable

Related to https://github.com/ArcBees/GWTP/issues/554
Can you reconsider your policy on final?

You're relying on a codestyle check to enforce a contract that can be enforced by a java modifier.  (Code Style for GWTP prevents reassigning parameters).

I think finals make code more readable.  I don't want to enforce them but if I use them I don't think it causes harm to future maintenance.