calculate method Null safety

Implementation

void calculate(Operation op, ArgDone arg) {
  try {
    op.pressed(this);
    final f = op.calcDisabled(controller) ? null : getCalculation(arg);
    if (f != null) {
      // Give the argument a chance to veto or defer the beforeCalculate
      // step.  This is needed for operations with a timeout, like the
      // 15C's STO to matrix.
      arg.handleOpBeforeCalculate(model, () => op.beforeCalculate(this));
      if (arg.liftStackIfEnabled(model)) {
        liftStackIfEnabled();
      }
      f(model);
      model.display.displayX();
      op.possiblyAlterStackLift(controller);
    }
  } on CalculatorError catch (e, stack) {
    controller.showCalculatorError(e, stack);
    // ignore: avoid_catches_without_on_clauses
  } catch (e, s) {
    debugPrint('Unexpected exception $e\n\n$s');
    controller.showCalculatorError(CalculatorError(9), s);
  }
}