registerConstraintUI(): Fix check for duplicate UI registrations

* Check for duplicates was based on "cstrClass" (of type Class), whereas
  the registration is based on the class name
* Note: This change does not affect the actual registration, hence no
  change in behavior is expected.
* Revealed by warning on unlikely argument of (type Class) for
  string2cstrMap.get()

Issue-Ref: 3539
Issue-Url: https://af3-developer.fortiss.org/issues/3539



Signed-off-by: Simon Barner <barner@fortiss.org>