Internally, ChoiceField expects both choices and preferred_choices to be a simple array, so I replaced incomplete bits of code that attempted to not modify a possible ArrayObject and instead added type checks in the configure() method (with unit tests for expected exceptions).
Property paths such as fields[group].fields[innerGroup].data were not being resolved correctly, since the second iteration of addError() (based on "group") would attempt to call get('fields') instead of get('innerGroup'). Solution is to remember to bump the propertyPath forward if we're at the fields property
