DragTarget crash if Draggable data field is not set, but data is not required.

[MousyBusiness]

DragTarget crashes if Draggable data field is not set, but data is not required.

Specifically the issue is in didDrop in drag_target.dart which asserts data is not null:

Suggestion

Make data field required.

[darshankawar]

@MousyBusiness
Can you provide flutter doctor -v and complete minimal code sample in text format that shows this issue ?
Thanks.

[MousyBusiness]

❯ flutter doctor -v
[✓] Flutter (Channel stable, 2.2.2, on macOS 11.4 20F71 darwin-x64, locale en-GB)
    • Flutter version 2.2.2 at /Users/mousybusiness/Flutter/flutter
    • Framework revision d79295af24 (7 days ago), 2021-06-11 08:56:01 -0700
    • Engine revision 91c9fc8fe0
    • Dart version 2.13.3

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/mousybusiness/Library/Android/sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.5, Build version 12E262
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.57.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension can be installed from:
      🔨 https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-x64     • macOS 11.4 20F71 darwin-x64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 91.0.4472.114

• No issues found!

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Draggable(
                // data: Colors.yellow,
                feedback: Container(height: 100, width: 100, color: Colors.green),
                child: Container(height: 100, width: 100, color: Colors.yellow.withOpacity(0.3))),
            Container(height: 100, width: 100, color: Colors.amber.withOpacity(0.3)),
            Container(height: 100, width: 100, color: Colors.red.withOpacity(0.3)),
            DragTarget(onWillAccept: (o) {
              return true;
            }, onAccept: (o) {
              print("onAccept");
            }, onLeave: (o) {
              print("onLeave");
            }, builder: (context, candidateData, rejectedData) {
              return Container(height: 100, width: 100, color: Colors.blue);
            }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {},
        child: Icon(Icons.add),
      ),
    );
  }
}

[MousyBusiness]

drag the first square onto the last square

[darshankawar]

Thanks for the details. I see the same behavior on latest master.

log

======== Exception caught by gesture library =======================================================
The following _CastError was thrown while routing a pointer event:
Null check operator used on a null value

When the exception was thrown, this was the stack: 
#0      _DragTargetState.didDrop (package:flutter/src/widgets/drag_target.dart:831:38)
#1      _DragAvatar.finishDrag (package:flutter/src/widgets/drag_target.dart:991:22)
#2      _DragAvatar.end (package:flutter/src/widgets/drag_target.dart:909:5)
#3      MultiDragPointerState._up (package:flutter/src/gestures/multidrag.dart:144:14)
#4      MultiDragGestureRecognizer._handleEvent (package:flutter/src/gestures/multidrag.dart:256:13)
#5      PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:94:12)
#6      PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:139:9)
#7      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:397:8)
#8      PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:137:18)
#9      PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:123:7)
#10     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:440:19)
#11     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:420:22)
#12     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:289:11)
#13     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:374:7)
#14     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:338:5)
#15     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:296:7)
#16     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:279:7)
#20     _invoke1 (dart:ui/hooks.dart:182:10)
#21     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:282:7)
#22     _dispatchPointerDataPacket (dart:ui/hooks.dart:96:31)
(elided 3 frames from dart:async)
====================================================================================================

flutter doctor -v

/Users/dhs/Documents/Fluttersdk/flutter/bin/flutter doctor --verbose
[✓] Flutter (Channel master, 2.3.0-17.0.pre.397, on Mac OS X 10.15.4 19E2269 darwin-x64, locale en-GB)
    • Flutter version 2.3.0-17.0.pre.397 at /Users/dhs/Documents/Fluttersdk/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 03034e9e4a (4 hours ago), 2021-06-18 00:14:03 -0700
    • Engine revision ced58ef9c1
    • Dart version 2.14.0 (build 2.14.0-217.0.dev)

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.3, Build version 12C33
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] VS Code (version 1.56.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.21.0

[✓] Connected device (3 available)
    • iPhone 12 Pro Max (mobile) • A5473606-0213-4FD8-BA16-553433949729 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-3 (simulator)
    • macOS (desktop)            • macos                                • darwin-x64     • Mac OS X 10.15.4 19E2269 darwin-x64
    • Chrome (web)               • chrome                               • web-javascript • Google Chrome 91.0.4472.106

• No issues found!