FAccordionController constructor
Creates a FAccordionController.
min and max define the minimum and maximum number of expanded items allowed.
Contract:
min and max must be: 0 <= min <= max.
Implementation
FAccordionController({int min = 0, int? max})
  : _controllers = {},
    _expanded = {},
    _min = min,
    _max = max,
    assert(debugCheckInclusiveRange<FAccordionController>(min, max));