Flutter null safety - The argument type 'Color?' can't be assigned to the parameter type 'Color'

Question

I changed my SDK version for flutter to min , so that I can fix my code for null safety.

There is one issue that I don't understand, this line produces the following error:

The argument type 'Color?' can't be assigned to the parameter type 'Color'

border: Border.all(color: Colors.grey[300], width: 1),

but if I change Colors.grey[300] with whatever value that doesn't use [], it will work, so Colors.grey works perfectly fine.

What should I change here to keep using grey[300]?

Solution 1

You can use 0xFFE0E0E0 for grey[300].

To pick material colors you can use this tool.

To select a specific color from one of the swatches, index into the swatch using an integer for the specific color desired, as follows:

Color selection = Colors.green[400]!; // Selects a mid-range green.

Each ColorSwatch constant is a color and can used directly. For example:

Container(
  color: Colors.blue, // same as Colors.blue[500] or Colors.blue.shade500
)

Solution 2

Problem:

Color color = Colors.grey[300]; // Error in null-safe mode

When you use Colors.grey[300], you're actually getting the color from a Map which Dart (in null-safe mode) warns you about because that value could be null. See more

---

Solutions:

There are two solutions for it. One is general and one is only specific to this case.

1. Use Bang operator (!)

   Color color = Colors.grey[300]!  // Added '!', error gone 
   

2. Use shadeXXX on the Color

   Color color = Colors.grey.shade300;
   

Solution 3

use shade300 like Color color = Colors.grey.shade300;

Comments

[AJ989]

yeah I noticed that, but does this mean using specific shades of a color with [x] is not allowed anymore since null safety in flutter 2?

[Simon Sot]

Well yeah, you can check issue here github.com/flutter/flutter/issues/71591#issue-755539014

[giorgio79]

Tried this, now I am getting "Invalid constant value"

[CopsOnRoad]

@giorgio79 Although it is unrelated but what color are you using?

[giorgio79]

Yeah, tried to use one of the shades. Turns out those are not constant, so have to use hex code Color(#123456...) lol. One of the bigger piles of dung in Flutter

[CopsOnRoad]

@giorgio79 But my answer doesn't have to do anything with it. You must already be getting the same error before using this answer. For example: Colors.grey[300] was already a non-const in the first place.

[Abdulhakim Zeinu]

Some description on the first solution. Use Bang operator ! only if you're sure that the object will never evaluated to null