Context: Not a constant expression, error on dart

Question

I was following a udemy course but I got stuck I got this error which I don't know why it ocurres

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  VoidCallback selectHandler;

  Answer(this.selectHandler);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: const RaisedButton(
        color: Colors.blue,
        child: Text('Answer 1'),
        onPressed: selectHandler,
      ),
    );
  }
}

onPressed: selectHandler, this line it the problematic one. I tried removing random keyword but i really don't know what to do.

that's the error log

lib/answer.dart:15:20: Error: Not a constant expression.
        onPressed: selectHandler,
                   ^^^^^^^^^^^^^
lib/answer.dart:12:20: Error: Constant evaluation error:
      child: const RaisedButton(
                   ^
lib/answer.dart:15:20: Context: Not a constant expression.
        onPressed: selectHandler,

Answer 1

You can't put functions inside const expressions, just remove the const keyword from:

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  VoidCallback selectHandler;

  Answer(this.selectHandler);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: /* const -> Remove this const keyword */ RaisedButton(
        color: Colors.blue,
        child: Text('Answer 1'),
        onPressed: selectHandler, // This is not a constant because the compiler can't assign the value at the compile time, so we can't use that in a const expression
      ),
    );
  }
}

Answer 2

You used const before RaisedButton when calling a variable inside the onPressed Function. Try removing the const keyword from the RaisedButton. It should work that way.

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  VoidCallback selectHandler;

  Answer(this.selectHandler);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: const RaisedButton(
        color: Colors.blue,
        child: Text('Answer 1'),
        onPressed: selectHandler,
      ),
    );
  }
}