A value of type 'Null' can't be assigned to a parameter of type 'String' in a const constructor

I'm unable to use questions[questionNumber] as a Text Constructor in Flutter.

Errors:

Evaluation of this constant expression throws an exception.dart(const_eval_throws_exception)

A value of type 'Null' can't be assigned to a parameter of type 'String' in a const constructor. Try using a subtype, or removing the keyword 'const'.dartconst_constructor_param_type_mismatch

Arguments of a constant creation must be constant expressions. Try making the argument a valid constant, or use 'new' to call the constructor.dartconst_with_non_constant_argument

class _QuizPageState extends State<QuizPage> {
  List<Widget> scoreKeeper = [];
  List<String> questions = [
    'You can lead a cow down stairs but not up stairs.',
    'Approximately one quarter of human bones are in the feet.',
    'A slug\'s blood is green.'
  ];

  int questionNumber = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        const Expanded(
          flex: 5,
          child: Padding(
            padding: EdgeInsets.all(10.0),
            child: Center(
              child: Text(
                questions[questionNumber],
                textAlign: TextAlign.center,
                style: TextStyle(
                  fontSize: 25.0,
                  color: Colors.white,
                ),
              ),
            ),
          ),
        ),
     ],
    );
  }
}