type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

[Kahoulam]

Steps to reproduce

1. Create a new project
2. Add tokens.json file
3. Execute the command dart run build_runner build

Expected results

Correctly generate tokens.dart file

Actual results

An error occurred:

type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

Version

design_tokens_builder: ^0.2.0+1

Code sample

tokens.json

{
  "global": {
    "M3": {
      "sys": {
        "light": {
          "primary": {
            "value": "#626200",
            "type": "color"
          },
          "on-primary": {
            "value": "#ffffff",
            "type": "color"
          },
          "primary-container": {
            "value": "#eaea00",
            "type": "color"
          },
          "on-primary-container": {
            "value": "#1d1d00",
            "type": "color"
          },
          "primary-fixed": {
            "value": "#eaea00",
            "type": "color"
          },
          "on-primary-fixed": {
            "value": "#1d1d00",
            "type": "color"
          },
          "primary-fixed-dim": {
            "value": "#cdcd00",
            "type": "color"
          },
          "on-primary-fixed-variant": {
            "value": "#494900",
            "type": "color"
          },
          "secondary": {
            "value": "#606043",
            "type": "color"
          },
          "on-secondary": {
            "value": "#ffffff",
            "type": "color"
          },
          "secondary-container": {
            "value": "#e7e4bf",
            "type": "color"
          },
          "on-secondary-container": {
            "value": "#1d1d06",
            "type": "color"
          },
          "secondary-fixed": {
            "value": "#e7e4bf",
            "type": "color"
          },
          "on-secondary-fixed": {
            "value": "#1d1d06",
            "type": "color"
          },
          "secondary-fixed-dim": {
            "value": "#cac8a5",
            "type": "color"
          },
          "on-secondary-fixed-variant": {
            "value": "#49482d",
            "type": "color"
          },
          "tertiary": {
            "value": "#3d6657",
            "type": "color"
          },
          "on-tertiary": {
            "value": "#ffffff",
            "type": "color"
          },
          "tertiary-container": {
            "value": "#bfecd8",
            "type": "color"
          },
          "on-tertiary-container": {
            "value": "#002117",
            "type": "color"
          },
          "tertiary-fixed": {
            "value": "#bfecd8",
            "type": "color"
          },
          "on-tertiary-fixed": {
            "value": "#002117",
            "type": "color"
          },
          "tertiary-fixed-dim": {
            "value": "#a4d0bd",
            "type": "color"
          },
          "on-tertiary-fixed-variant": {
            "value": "#254e40",
            "type": "color"
          },
          "error": {
            "value": "#ba1a1a",
            "type": "color"
          },
          "on-error": {
            "value": "#ffffff",
            "type": "color"
          },
          "error-container": {
            "value": "#ffdad6",
            "type": "color"
          },
          "on-error-container": {
            "value": "#410002",
            "type": "color"
          },
          "outline": {
            "value": "#797869",
            "type": "color"
          },
          "background": {
            "value": "#fffbff",
            "type": "color"
          },
          "on-background": {
            "value": "#1c1c17",
            "type": "color"
          },
          "surface": {
            "value": "#fdf9f0",
            "type": "color"
          },
          "on-surface": {
            "value": "#1c1c17",
            "type": "color"
          },
          "surface-variant": {
            "value": "#e6e3d1",
            "type": "color"
          },
          "on-surface-variant": {
            "value": "#48473a",
            "type": "color"
          },
          "inverse-surface": {
            "value": "#31312b",
            "type": "color"
          },
          "inverse-on-surface": {
            "value": "#f4f0e8",
            "type": "color"
          },
          "inverse-primary": {
            "value": "#cdcd00",
            "type": "color"
          },
          "shadow": {
            "value": "#000000",
            "type": "color"
          },
          "surface-tint": {
            "value": "#626200",
            "type": "color"
          },
          "outline-variant": {
            "value": "#cac7b6",
            "type": "color"
          },
          "scrim": {
            "value": "#000000",
            "type": "color"
          },
          "surface-container-highest": {
            "value": "#e6e2d9",
            "type": "color"
          },
          "surface-container-high": {
            "value": "#ebe8df",
            "type": "color"
          },
          "surface-container": {
            "value": "#f1eee5",
            "type": "color"
          },
          "surface-container-low": {
            "value": "#f7f3ea",
            "type": "color"
          },
          "surface-container-lowest": {
            "value": "#ffffff",
            "type": "color"
          },
          "surface-bright": {
            "value": "#fdf9f0",
            "type": "color"
          },
          "surface-dim": {
            "value": "#dddad1",
            "type": "color"
          }
        }
      }
    }
  },
  "$themes": [],
  "$metadata": {
    "tokenSetOrder": [
      "global"
    ]
  }
}

Logs

Console logs

[INFO] Generating build script completed, took 235ms
[INFO] Reading cached asset graph completed, took 64ms
[INFO] Checking for updates since last build completed, took 843ms
[WARNING] design_tokens_builder|lib/design_tokens_builder.dart was not found in the asset graph, incremental builds will not work.
 This probably means you don't have your dependencies specified fully in your pubspec.yaml.
[WARNING] Invalidating asset graph due to build script update!
[INFO] Cleaning up outputs from previous builds. completed, took 1ms
[INFO] Generating build script completed, took 40ms
[WARNING] Invalidated precompiled build script due to missing asset graph.
[INFO] Precompiling build script... completed, took 596ms
[WARNING] design_tokens_builder|lib/design_tokens_builder.dart was not found in the asset graph, incremental builds will not work.
 This probably means you don't have your dependencies specified fully in your pubspec.yaml.
[INFO] Building new asset graph completed, took 900ms
[INFO] Checking for unexpected pre-existing outputs. completed, took 0ms
[SEVERE] design_tokens_builder on lib/$lib$:

type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast
[INFO] Running build completed, took 100ms
[INFO] Caching finalized dependency graph completed, took 39ms
[SEVERE] Failed after 145ms

Flutter Doctor output

Doctor output

  [✓] Flutter (Channel stable, 3.13.9, on macOS 14.0 23A344 darwin-arm64, locale en-TW)
  [✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
  [✓] Xcode - develop for iOS and macOS (Xcode 15.0.1)
  [✓] Chrome - develop for the web
  [✓] Android Studio (version 2022.3)
  [✓] VS Code (version 1.84.0)
  [✓] Connected device (1 available)
  [✓] Network resources