When I try to define the list of colors I get the following message: The element type 'Color?' can't be assigned to the list type 'Color'

return Scaffold(
      body: Container(
        padding: EdgeInsets.symmetric(vertical:30.0),
        width: double.infinity,
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.topCenter,
            colors: [
              Colors.orange[900],
              Colors.orange[800]
            ]
          )
        ),
      ),
    );

I need help please

CodePudding user response：

When using a subscript to get a color from a ColorSwatch, you will get a Color? which can possibly be a null value. That's because there's no such value as, for example, Colors.orange[1234] - So that would be a null value.

The colors list on the LinearGradient, on the other hand, requires all non-nullable values.

In your case, you're safe simply asserting that the values are non-null, since the colors you're referring to definitely exist:

gradient: LinearGradient(
  begin: Alignment.topCenter,
  colors: [
    Colors.orange[900]!,
    Colors.orange[800]!
  ]
)

You can also avoid the subscripts altogether, like so:

gradient: LinearGradient(
  begin: Alignment.topCenter,
  colors: [
    Colors.orange.shade900,
    Colors.orange.shade800,
  ]
)

Possibly helpful links:

MaterialColor

Sound Null Safety

CodePudding user response：

Whenever working with gradients.. i better approach would be to use Colors by providing their hex code.. In flutter when you write Colors. it shows you a list of colors and their shades with their hex code.. https://api.flutter.dev/flutter/material/Colors-class.html

Like in this link. So update your code like this

gradient: LinearGradient( begin: Alignment.topCenter, colors: [ const Color(0xFFE65100), const Color(0xFFEF6C00), ] )

using hex color code is better because their value remain constant and when state is changed that part remains constant (doesn't rebuild)

Do upvote if useful