Flutter ,dart,The parameter 'colour' can't have a value of 'null' because of its type, but the implicit default value is 'null'

Question

i dont know why i am getting the errors,i have previously coded the same and it worked perfectly fine maybe some new update or plugin please help me solve my error

import 'package:flutter/material.dart';

class RoundedButton extends StatelessWidget {
  final String title;
  final Color colour;
  final Function onPressed;
  RoundedButton({this.title, this.colour, @required this.onPressed});

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.symmetric(vertical: 16.0),
      child: Material(
        elevation: 5.0,
        color: colour,
        borderRadius: BorderRadius.circular(30.0),
        child: MaterialButton(
          onPressed: onPressed,
          minWidth: 200.0,
          height: 42.0,
          child: Text(
            title,
            style: TextStyle(color: Colors.white),
          ),
        ),
      ),
    );
  }
}

Answer 1

You've updated your Dart SDK to a version which supports null-safety. With null safety there are some differences to the way constructors work in Dart.

1. @required is now simply required - No @ symbol anymore.
2. If a property is not required, then it must have either a default value, or it must have a nullable type, e.g. String? or Color?.

Using default values, your code might look something like this:

final String title;
final Color colour;
final void Function() onPressed;

RoundedButton({
    this.title = "",
    this.colour = Colors.blue,
    required this.onPressed,
});

The rest of your class would be the same.

---

On the other hand, if you make the properties nullable, you might end up with something like this:

final String? title;
final Color? colour;
final void Function() onPressed;

RoundedButton({
    this.title,
    this.colour,
    required this.onPressed,
});

Now, for the colour property, you won't need to change anything, since Material.color can be null. However, since Text requires a non-nullable String, you'll have to change the way you pass the title property:

Text(
    title ?? "", // If title is null, use an empty string
    // ...
)

or

child: title == null // Only build the Text widget if title is not null
    ? null 
    : Text(
          title,
          // ...
      )

In depth reading on null safety here: https://dart.dev/null-safety/understanding-null-safety