handleLetterLabel method Null safety

Implementation

@override
void handleLetterLabel(LetterLabel operation) {
  final program = model.memory.program;
  assert(controller is RealController);
  // The 15C's letter labels get translated to GSB <label> when entering
  // a program, so they cannot occur if we're running a program (i.e.
  // when controller is RunningController).  The return stack should only
  // be reset when we're not currently running a program, which is the only
  // case here.
  program.resetReturnStack();
  final gsb = controller.gsbOperation;
  try {
    program.gosub(operation.numericValue);
  } on CalculatorError catch (e, s) {
    controller.showCalculatorError(e, s);
    return;
  }
  program.displayCurrent();
  final s = RunProgramArgInputState(
      gsb, gsb.arg, controller, this, () => GosubProgramRunner());
  s.isDone = true;
  // Since we're telling the state it's done, it won't try to do the gosub.
  // It will just wait for button up, then switch to gosubDoneState, which
  // will run the program.
  changeState(s);
}