Fixes `DragTarget` crash if `Draggable.data` is `null` (#133136)

Makes the `data` parameter of `Draggable` non-nullable.

Fixes https://github.com/flutter/flutter/issues/84816

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

packages/flutter/lib/src/widgets/drag_target.dart

@@ -652,11 +652,13 @@ class DragTarget<T extends Object> extends StatefulWidget {
   final DragTargetWillAcceptWithDetails<T>? onWillAcceptWithDetails;
 
   /// Called when an acceptable piece of data was dropped over this drag target.
+  /// It will not be called if `data` is `null`.
   ///
   /// Equivalent to [onAcceptWithDetails], but only includes the data.
   final DragTargetAccept<T>? onAccept;
 
   /// Called when an acceptable piece of data was dropped over this drag target.
+  /// It will not be called if `data` is `null`.
   ///
   /// Equivalent to [onAccept], but with information, including the data, in a
   /// [DragTargetDetails].
@@ -666,7 +668,8 @@ class DragTarget<T extends Object> extends StatefulWidget {
   /// the target.
   final DragTargetLeave<T>? onLeave;
 
-  /// Called when a [Draggable] moves within this [DragTarget].
+  /// Called when a [Draggable] moves within this [DragTarget]. It will not be
+  /// called if `data` is `null`.
   ///
   /// This includes entering and leaving the target.
   final DragTargetMove<T>? onMove;
@@ -707,6 +710,7 @@ class _DragTargetState<T extends Object> extends State<DragTarget<T>> {
                                     (widget.onWillAccept != null &&
                                     widget.onWillAccept!(avatar.data as T?)) ||
                                     (widget.onWillAcceptWithDetails != null &&
+                                    avatar.data != null &&
                                     widget.onWillAcceptWithDetails!(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!)));
     if (resolvedWillAccept) {
       setState(() {
@@ -741,12 +745,14 @@ class _DragTargetState<T extends Object> extends State<DragTarget<T>> {
     setState(() {
       _candidateAvatars.remove(avatar);
     });
+    if (avatar.data != null)  {
       widget.onAccept?.call(avatar.data! as T);
       widget.onAcceptWithDetails?.call(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!));
     }
+  }
 
   void didMove(_DragAvatar<Object> avatar) {
-    if (!mounted) {
+    if (!mounted || avatar.data == null) {
       return;
     }
     widget.onMove?.call(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!));

packages/flutter/test/widgets/draggable_test.dart

@@ -395,6 +395,47 @@ void main() {
     expect(targetMoveCount['Target 2'], equals(1));
   });
 
+  testWidgetsWithLeakTracking('Drag and drop - onMove is not called if moved with null data', (WidgetTester tester) async {
+    bool onMoveCalled = false;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Column(
+        children: <Widget>[
+          const Draggable<int>(
+            feedback: Text('Dragging'),
+            child: Text('Source'),
+          ),
+          DragTarget<int>(
+            builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
+              return const SizedBox(height: 100.0, child: Text('Target'));
+            },
+            onMove: (DragTargetDetails<dynamic> details) {
+              onMoveCalled = true;
+            },
+          ),
+        ],
+      ),
+    ));
+
+    expect(onMoveCalled, isFalse);
+
+    final Offset firstLocation = tester.getCenter(find.text('Source'));
+    final TestGesture gesture = await tester.startGesture(firstLocation, pointer: 7);
+    await tester.pump();
+
+    expect(onMoveCalled, isFalse);
+
+    final Offset secondLocation = tester.getCenter(find.text('Target'));
+    await gesture.moveTo(secondLocation);
+    await tester.pump();
+
+    expect(onMoveCalled, isFalse);
+    await gesture.up();
+    await tester.pump();
+
+    expect(onMoveCalled, isFalse);
+  });
+
   testWidgetsWithLeakTracking('Drag and drop - dragging over button', (WidgetTester tester) async {
     final List<String> events = <String>[];
     Offset firstLocation, secondLocation;
@@ -2392,6 +2433,68 @@ void main() {
     expect(find.text('Target'), findsOneWidget);
   });
 
+  testWidgetsWithLeakTracking('Drag and drop - onAccept is not called if dropped with null data', (WidgetTester tester) async {
+    bool onAcceptCalled = false;
+    bool onAcceptWithDetailsCalled = false;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Column(
+        children: <Widget>[
+          const Draggable<int>(
+            feedback: Text('Dragging'),
+            child: Text('Source'),
+          ),
+          DragTarget<int>(
+            builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
+              return const SizedBox(height: 100.0, child: Text('Target'));
+            },
+            onAccept: (int data) {
+              onAcceptCalled = true;
+            },
+            onAcceptWithDetails: (DragTargetDetails<int> details) {
+              onAcceptWithDetailsCalled =true;
+            },
+          ),
+        ],
+      ),
+    ));
+
+    expect(onAcceptCalled, isFalse);
+    expect(onAcceptWithDetailsCalled, isFalse);
+    expect(find.text('Source'), findsOneWidget);
+    expect(find.text('Dragging'), findsNothing);
+    expect(find.text('Target'), findsOneWidget);
+
+    final Offset firstLocation = tester.getCenter(find.text('Source'));
+    final TestGesture gesture = await tester.startGesture(firstLocation, pointer: 7);
+    await tester.pump();
+
+    expect(onAcceptCalled, isFalse);
+    expect(onAcceptWithDetailsCalled, isFalse);
+    expect(find.text('Source'), findsOneWidget);
+    expect(find.text('Dragging'), findsOneWidget);
+    expect(find.text('Target'), findsOneWidget);
+
+    final Offset secondLocation = tester.getCenter(find.text('Target'));
+    await gesture.moveTo(secondLocation);
+    await tester.pump();
+
+    expect(onAcceptCalled, isFalse);
+    expect(onAcceptWithDetailsCalled, isFalse);
+    expect(find.text('Source'), findsOneWidget);
+    expect(find.text('Dragging'), findsOneWidget);
+    expect(find.text('Target'), findsOneWidget);
+
+    await gesture.up();
+    await tester.pump();
+
+    expect(onAcceptCalled, isFalse, reason: 'onAccept should not be called when data is null');
+    expect(onAcceptWithDetailsCalled, isFalse, reason: 'onAcceptWithDetails should not be called when data is null');
+    expect(find.text('Source'), findsOneWidget);
+    expect(find.text('Dragging'), findsNothing);
+    expect(find.text('Target'), findsOneWidget);
+  });
+
   testWidgetsWithLeakTracking('Draggable disposes recognizer', (WidgetTester tester) async {
     late final OverlayEntry entry;
     addTearDown(() => entry..remove()..dispose());