type 'Null' is not a subtype of type 'String'

[jeremielodi]

I Suggest to have the file name and the line where has error happened. It's very complicated to know directly where to fixe the type when you face this error.

[bwilkerson]

Which tool are you using when you see this message without any information about where the error happened?

[jeremielodi]

[jeremielodi]

Even with android studio or VS code

[bwilkerson]

I can reproduce the problem in DartPad, but when I run

void main() {
  Map<String, dynamic> map = {"name": "jeremie", "age": 12};
  String sex = map['sex'];
  print(sex);
}

in VSCode, the following output is printed in the Debug Console:

Unhandled exception:
type 'Null' is not a subtype of type 'String'
#0      main (package:test_package/test.dart:3:10)
#1      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#2      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

Exited (255).

The stack trace does indicate where the exception was thrown (on the line marked with #0).

I haven't tried to reproduce it in Android Studio, but I'd be surprised if it didn't provide you with the full stack trace.

[bwilkerson]

@johnpryan

[jeremielodi]

I think, I didn't reproduce the issue correctly,
This come to me when I try to get data from http response. If the key doesn't exist in the map or the type doesn't much then this error occured.

[bwilkerson]

Thanks for the additional information.

This is definitely an issue for DartPad, even without hot reload, but it appears to also be an issue for hot reload.

[johnpryan]

I filed this issue for DartPad: dart-lang/dart-pad#2668

[johnpryan]

@bwilkerson Off topic, but do you know why the Flutter SDK prints a different error message in this scenario? Here's what is printed when the exception is thrown from a Flutter app:

The following TypeErrorImpl was thrown while handling a gesture:
Expected a value of type 'String', but got one of type 'Null'

And here's what the exception looks like if you use a Dart SDK:

type 'Null' is not a subtype of type 'String'

[bwilkerson]

No, I don't know why they're different. Obviously Flutter is catching the exception at some point and wrapping the message, but I would have expected the second line of Flutter's message to match what the Dart SDK is producing.

Is it possible that you're using two different versions of the Dart SDK somehow? If not, I have no clue.

[johnpryan]

@jeremielodi Can you share a small snippet that reproduces the issue? I can't reproduce this issue in Flutter after hot-reloading. I put the code you shared in the _incrementCounter() function in the Counter app and I'm seeing a correct looking error message:

Snippet

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    Map<String, dynamic> map = {"name": "jeremie", "age": 12};
    String sex = map['sex'];
    print(sex);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

======== Exception caught by gesture ===============================================================
The following _TypeError was thrown while handling a gesture:
type 'Null' is not a subtype of type 'String'

When the exception was thrown, this was the stack: 
#0      _MyHomePageState._incrementCounter (package:foo/main.dart:37:12)
#1      _InkResponseState.handleTap (package:flutter/src/material/ink_well.dart:1154:21)
#2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:275:24)
#3      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:654:11)
#4      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:311:5)
#5      BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:244:7)
#6      PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:630:9)
#7      PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:98:12)
#8      PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:143:9)
#9      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:625:13)
#10     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:141:18)
#11     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:127:7)
#12     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:488:19)
#13     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:468:22)
#14     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:333:11)
#15     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:413:7)
#16     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:376:5)
#17     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:323:7)
#18     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:292:9)
#19     _invoke1 (dart:ui/hooks.dart:186:13)
#20     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:424:7)
#21     _dispatchPointerDataPacket (dart:ui/hooks.dart:119:31)
Handler: "onTap"
Recognizer: TapGestureRecognizer#e9d5f
  debugOwner: GestureDetector
  state: possible
  won arena
  finalPosition: Offset(773.1, 562.9)
  finalLocalPosition: Offset(45.1, 34.9)
  button: 1
  sent tap down
====================================================================================================

[github-actions]

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[eernstg]

I guess the real topic here is the lack of stack traces and other information. However, it's worth noting that the example is working as intended:

void main() {
  Map<String, dynamic> map = {"name": "jeremie", "age": 12};
  String sex = map['sex']; // Throws.
  print(sex);
}

The exception is thrown when the line with the comment is reached because the type of the expression map['sex'] is dynamic, and expressions with that type are subject to type checks at run time. The value of an expression of type dynamic can be assigned to anything, but it will be checked at run time that the given value has the required type, in this case it must be a String. However, the value is actually null. So the dynamic type check fails and an exception is thrown.

Nothing surprising about this, and everything is working as intended.

However, if you wish to avoid this kind of dynamic type check then you can make sure that you have an 'analysis_options.yaml' file in the root of your package that contains at least the following:

analyzer:
  language:
    strict-casts: true

You'll find some extra information about these settings here.

[a-balakmet]

I had the same exception with code below:

class Place { final int id; final String name; final String? address; }
I solved it like this:

class Place { final int id; final String name; final dynamic address; }