init method Null safety

Implementation

@override
void init(int registerBase,
    {required OpInitFunction f,
    required ProgramOperation? shift,
    required bool argDot,
    required ProgramOperation? arg,
    required bool userMode}) {
  if (arg != null) {
    // Move arg over to shift, e.g. for STO + 7
    assert(shift == null);
    shift = arg;
    arg = null;
  }
  _next = List.generate(min(max + 1, registerBase), (i) {
    final a = argDoneFactory((m) => calc(m, i)); // Function currying, baby
    a.init(registerBase,
        f: f,
        shift: shift,
        argDot: argDot,
        arg: Arg.kDigits[i],
        userMode: userMode);
    return a;
  }, growable: false);
  if (max < registerBase) {
    _nextOnDot = null;
  } else {
    assert(!argDot, 'max: $max');
    final ka = _nextOnDot = KeyArg(
        key: Arg.kDot,
        child: DigitArg(
            max: max - registerBase,
            calc: (m, i) => calc(m, i + registerBase),
            argDoneFactory: argDoneFactory));
    // Skip over ka, straight to its child:
    ka.child.init(registerBase,
        f: f, shift: shift, argDot: true, arg: arg, userMode: userMode);
    // argName: '.', userMode: userMode));
  }
}