Null check operator used on a null value after pressing Done on the keyboard in flutter

Please tell me, I have a problem with textfield. When I finished entering data in the field, I press the Done key on the keyboard and I get this error Null check operator used on a null value
although all data is saved. And which operator has a null value, I can’t understand, after all, everything is saved. How can I remove this error or warning?

text_field

class PriceCounter extends StatefulWidget {
  PriceCounter({Key? key, required this.price, this.onChanged})
      : super(key: key);
  double price;
  final Function(double)? onChanged;

  @override
  State<PriceCounter> createState() => _PriceCounterState();
}

class _PriceCounterState extends State<PriceCounter> {
  final _priceController = TextEditingController();

  @override
  void dispose() {
    _priceController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final FilterPriceCubit cubit = BlocProvider.of<FilterPriceCubit>(context);

    return BlocBuilder<FilterPriceCubit, FilterPriceState>(
      builder: (context, state) {
        _priceController.text = state.fitlerPrice.toStringAsFixed(2).toString();
        return Padding(
          padding: const EdgeInsets.symmetric(horizontal: 21),
          child: Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              IconButton(
                onPressed: () => cubit.priceFilterDecrement(state.fitlerPrice),
                icon: SvgPicture.asset(constants.Assets.minus),
                constraints: const BoxConstraints(),
                padding: EdgeInsets.zero,
              ),
              const SizedBox(width: 20),
              SizedBox(
                width: 100,
                child: TextFormField(
                  keyboardType: TextInputType.number,
                  // controller: _priceController
                  //   ..text = widget.price.toStringAsFixed(2),
                  controller: _priceController,
                  style: constants.Styles.normalBookTextStyleWhite,
                  textAlign: TextAlign.center,
                  decoration: const InputDecoration(
                    prefix: Text('JC',
                        style: constants.Styles.normalBookTextStyleWhite),
                    suffix: Text(
                      'KWh',
                      style: constants.Styles.smallerBookTextStyleWhite,
                    ),
                    contentPadding: EdgeInsets.zero,
                    border: InputBorder.none,
                  ),
                  onFieldSubmitted: (value) {
                    cubit.setPrice(value);
                    widget.onChanged!(double.parse(value));
                  },

error

>Solution :

An error occurs at this point:

widget.onChanged!(double.parse(value));

You have to be sure that widget.onChanged isn’t null.

The best way is:

if (widget.onChanged != null) {
   widget.onChanged!(double.parse(value));
}

or

widget.onChanged?.call(double.parse(value));