Some troubleshooting steps if you're running into issues loading assets in your Flutter app, either with rootBundle.loadString() or something like Image.asset().

You haven't registered your assets

Symptom

You get the following error message

Unable to load asset

Cause

Your asset hasn't been properly declared in your pubspec.yaml file. This can either be due to not specifying the asset at all, or indenting incorrectly.

Open pubspec.yaml, and find the flutter.assets section. You want to ensure that you are specifying each item under assets as an array element (prefixed with a hyphen, indented).

Notes:

• Whole folders can be specified by ensuring your item ends with a slash, however assets will only be pulled from that one level (i.e. asset declarations are not fully recursive).
• Further sub-directories can be used for image variants (such as dark equivalents, or relative sizes of images). See the Flutter documentation for more information on this.
• Pathing is relative to the root of the project, so if you put all of your assets in an assets/ directory, then every asset declaration must be prefixed with assets/.

Example:

flutter:
  assets:
    # Further indented by two spaces, with a hyphen at the start of each
    - assets/images/logo.png # A single file included
    - assets/images/splash/ # A directory containing several files
    

ServicesBinding.defaultBinaryMessenger was not initialized yet

Symptom

You get the following unhandled exception:

E/flutter (12359): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter (12359): #0      PlatformAssetBundle.load (package:flutter/src/services/asset_bundle.dart:222:39)
E/flutter (12359): #1      AssetBundle.loadString (package:flutter/src/services/asset_bundle.dart:68:33)
E/flutter (12359): #2      CachingAssetBundle.loadString.<anonymous closure> (package:flutter/src/services/asset_bundle.dart:165:56)
E/flutter (12359): #3      _LinkedHashMapMixin.putIfAbsent (dart:collection-patch/compact_hash.dart:314:23)
E/flutter (12359): #4      CachingAssetBundle.loadString (package:flutter/src/services/asset_bundle.dart:165:27)

Cause

The widget library hasn't yet been initialized. This can happen if you are trying to load an asset before your app has fully initialized.

Solution

Put the following code into your application. There's one import, then one line of code right before you need to run rootBundle.loadString()

// Import
import 'package:flutter/widgets.dart' show WidgetsFlutterBinding;

// Right before you would be doing any loading
WidgetsFlutterBinding.ensureInitialized();

// Load the asset you wanted to load
final configFile = await rootBundle.loadString('assets/env/development.json');

Further Reading

Adding assets and images

How to use images (and other assets) in your Flutter app.

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