More gracefully handle changing selection from bean class to bean reference in property page

Currently if you define a new global bean, then change it from either a referenced factory bean to a concrete bean class or vice versa, you end up with both fields populated, which creates an error: "Must specify either an explicit class name in the project or a reference to a global bean that exposes one, not both."

The workaround is to explicitly clear one field, select another bean (or global entry) and reselect the bean to clear the error.