The argument type 'String?' can't be assigned to the parameter type 'String'

This is my code

Map<String, String> coinValues = {};
bool isWaiting = false;
void getData() async {
  isWaiting = true;
  try {
    var data = await CoinData().getCoinData(selectedCurrency);

    isWaiting = false;

    setState(() {
      coinValues = data;
    });
  } catch (e) {
    print(e);
  }
}

Future<Column> makeCards() async {
  List<CryptoCard> cryptoCards = [];
  for (String crypto in cryptoList) {
    cryptoCards.add(
      CryptoCard(
        cryptoCurrency: crypto,
        selectedCurrency: selectedCurrency,
        value: isWaiting ? '?' : coinValues[crypto],
      ),
    );
  }
  return Column(
    crossAxisAlignment: CrossAxisAlignment.stretch,
    children: cryptoCards,
  );
}

I get the following error

Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
          value: isWaiting ? '?' : coinValues[crypto],

I tried adding a null check but the app crash and says "Null check operator used on a null value"

>Solution :

It means that your coinValues[crypto] returns null.

You should check if coinValues[crypto] could be null or not.

Otherwise you can try the following code :

Future<Column> makeCards() async {
  List<CryptoCard> cryptoCards = [];
  for (String crypto in cryptoList) {
    cryptoCards.add(
      CryptoCard(
        cryptoCurrency: crypto,
        selectedCurrency: selectedCurrency,
        value: isWaiting || coinValues[crypto] == null ? '?' : coinValues[crypto]!,
      ),
    );
  }
  return Column(
    crossAxisAlignment: CrossAxisAlignment.stretch,
    children: cryptoCards,
  );
}