stepWidget property
The widget (UI representation) of the step.
RPQuestionSteps don't have it because their UI representation depends
on the Answer Format.
Their Answer Format is assessed by RPUIQuestionStep or RPUIFormStep and
they return the corresponding UI.
When needed, it should be overridden to fit the different types of Steps.
Implementation
@override
Widget get stepWidget => ExtensionInstructionsStepUI(step: this);