Revert "Draggable can be accepted when the data is null" (#99419)

dev/manual_tests/lib/drag_and_drop.dart

@@ -16,9 +16,9 @@ class ExampleDragTarget extends StatefulWidget {
 class ExampleDragTargetState extends State<ExampleDragTarget> {
   Color _color = Colors.grey;
 
-  void _handleAccept(Color? data) {
+  void _handleAccept(Color data) {
     setState(() {
-      _color = data!;
+      _color = data;
     });
   }
 
@@ -215,7 +215,7 @@ class MovableBall extends StatelessWidget {
       );
     } else {
       return DragTarget<bool>(
-        onAccept: (bool? data) { callback(position); },
+        onAccept: (bool data) { callback(position); },
         builder: (BuildContext context, List<bool?> accepted, List<dynamic> rejected) {
           return dashedBall;
         },

examples/api/lib/widgets/drag_target/draggable.0.dart

@@ -81,9 +81,9 @@ class _MyStatefulWidgetState extends State<MyStatefulWidget> {
               ),
             );
           },
-          onAccept: (int? data) {
+          onAccept: (int data) {
             setState(() {
-              acceptedData += data!;
+              acceptedData += data;
             });
           },
         ),

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

@@ -20,7 +20,7 @@ typedef DragTargetWillAccept<T> = bool Function(T? data);
 /// Signature for causing a [DragTarget] to accept the given data.
 ///
 /// Used by [DragTarget.onAccept].
-typedef DragTargetAccept<T> = void Function(T? data);
+typedef DragTargetAccept<T> = void Function(T data);
 
 /// Signature for determining information about the acceptance by a [DragTarget].
 ///
@@ -638,7 +638,7 @@ class DragTargetDetails<T> {
   DragTargetDetails({required this.data, required this.offset}) : assert(offset != null);
 
   /// The data that was dropped onto this [DragTarget].
-  final T? data;
+  final T data;
 
   /// The global position when the specific pointer event occurred on
   /// the draggable.
@@ -767,14 +767,14 @@ class _DragTargetState<T extends Object> extends State<DragTarget<T>> {
     setState(() {
       _candidateAvatars.remove(avatar);
     });
-    widget.onAccept?.call(avatar.data as T?);
-    widget.onAcceptWithDetails?.call(DragTargetDetails<T>(data: avatar.data as T?, offset: avatar._lastOffset!));
+    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)
       return;
-    widget.onMove?.call(DragTargetDetails<T>(data: avatar.data as T?, offset: avatar._lastOffset!));
+    widget.onMove?.call(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!));
   }
 
   @override