IMO, if ```getActiveSubClass``` fallbacks to ```getClass``` method, we need to redefine ```getFormBuilder``` and ```getNewInstance``` methods which use the same trick ```$this->getActiveSubClass() ?: $this->getClass()```.
And what about ```getActiveSubClassCode``` which returns a code or null ?

Maybe the most logic should be to redefine the the ```getClass```and remove  the ```getActiveSubClass``` method. 
What's the benefit of keeping different accessors for ```class``` and ```activeSubClass``` ?