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

Well, the error is due to using the keyword const for the Expanded widget. Just remove it and you will be all good.

So this:

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        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,
                ),
              ),
            ),
          ),
        ),
     ],
    );
  }
}